# Multivector Fields¶

Let $$U$$ and $$M$$ be two differentiable manifolds. Given a positive integer $$p$$ and a differentiable map $$\Phi: U \rightarrow M$$, a multivector field of degree $$p$$, or $$p$$-vector field, along $$U$$ with values on $$M$$ is a field along $$U$$ of alternating contravariant tensors of rank $$p$$ in the tangent spaces to $$M$$. The standard case of a multivector field on a differentiable manifold corresponds to $$U = M$$ and $$\Phi = \mathrm{Id}_M$$. Other common cases are $$\Phi$$ being an immersion and $$\Phi$$ being a curve in $$M$$ ($$U$$ is then an open interval of $$\RR$$).

Two classes implement multivector fields, depending whether the manifold $$M$$ is parallelizable:

AUTHORS:

• Eric Gourgoulhon (2017): initial version

REFERENCES:

class sage.manifolds.differentiable.multivectorfield.MultivectorField(vector_field_module, degree, name=None, latex_name=None)

Multivector field with values on a generic (i.e. a priori not parallelizable) differentiable manifold.

Given a differentiable manifold $$U$$, a differentiable map $$\Phi: U \rightarrow M$$ to a differentiable manifold $$M$$ and a positive integer $$p$$, a multivector field of degree $$p$$ (or $$p$$-vector field) along $$U$$ with values on $$M\supset\Phi(U)$$ is a differentiable map

$a:\ U \longrightarrow T^{(p,0)}M$

($$T^{(p,0)}M$$ being the tensor bundle of type $$(p,0)$$ over $$M$$) such that

$\forall x \in U,\quad a(x) \in \Lambda^p(T_{\Phi(x)} M) ,$

where $$T_{\Phi(x)} M$$ is the vector space tangent to $$M$$ at $$\Phi(x)$$ and $$\Lambda^p$$ stands for the exterior power of degree $$p$$ (cf. ExtPowerFreeModule). In other words, $$a(x)$$ is an alternating contravariant tensor of degree $$p$$ of the tangent vector space $$T_{\Phi(x)} M$$.

The standard case of a multivector field on a manifold $$M$$ corresponds to $$U = M$$ and $$\Phi = \mathrm{Id}_M$$. Other common cases are $$\Phi$$ being an immersion and $$\Phi$$ being a curve in $$M$$ ($$U$$ is then an open interval of $$\RR$$).

Note

If $$M$$ is parallelizable, the class MultivectorFieldParal must be used instead.

INPUT:

• vector_field_module – module $$\mathfrak{X}(U,\Phi)$$ of vector fields along $$U$$ with values on $$M$$ via the map $$\Phi$$
• degree – the degree of the multivector field (i.e. its tensor rank)
• name – (default: None) name given to the multivector field
• latex_name – (default: None) LaTeX symbol to denote the multivector field; if none is provided, the LaTeX symbol is set to name

EXAMPLES:

Multivector field of degree 2 on a non-parallelizable 2-dimensional manifold:

sage: M = Manifold(2, 'M')
sage: U = M.open_subset('U') ; V = M.open_subset('V')
sage: M.declare_union(U,V)   # M is the union of U and V
sage: c_xy.<x,y> = U.chart() ; c_uv.<u,v> = V.chart()
sage: xy_to_uv = c_xy.transition_map(c_uv, (x+y, x-y),
....:                         intersection_name='W',
....:                         restrictions1= x>0, restrictions2= u+v>0)
sage: uv_to_xy = xy_to_uv.inverse()
sage: W = U.intersection(V)
sage: eU = c_xy.frame() ; eV = c_uv.frame()
sage: a = M.multivector_field(2, name='a') ; a
2-vector field a on the 2-dimensional differentiable manifold M
sage: a.parent()
Module A^2(M) of 2-vector fields on the 2-dimensional differentiable
manifold M
sage: a.degree()
2


Setting the components of a:

sage: a[eU,0,1] = x*y^2 + 2*x
sage: a.display(eU)
a = (x*y^2 + 2*x) d/dx/\d/dy
sage: a.display(eV)
a = (-1/4*u^3 + 1/4*u*v^2 - 1/4*v^3 + 1/4*(u^2 - 8)*v - 2*u) d/du/\d/dv


It is also possible to set the components while defining the 2-vector field definition, via a dictionary whose keys are the vector frames:

sage: a1 = M.multivector_field(2, {eU: [[0, x*y^2 + 2*x],
....:                                   [-x*y^2 - 2*x, 0]]}, name='a')
sage: a1 == a
True


The exterior product of two vector fields is a 2-vector field:

sage: a = M.vector_field({eU: [-y, x]}, name='a')
sage: b = M.vector_field({eU: [1+x*y, x^2]}, name='b')
sage: s = a.wedge(b) ; s
2-vector field a/\b on the 2-dimensional differentiable manifold M
sage: s.display(eU)
a/\b = (-2*x^2*y - x) d/dx/\d/dy
sage: s.display(eV)
a/\b = (1/2*u^3 - 1/2*u*v^2 - 1/2*v^3 + 1/2*(u^2 + 2)*v + u) d/du/\d/dv


Multiplying a 2-vector field by a scalar field results in another 2-vector field:

sage: f = M.scalar_field({c_xy: (x+y)^2, c_uv: u^2}, name='f')
sage: s = f*s ; s
2-vector field f*(a/\b) on the 2-dimensional differentiable manifold M
sage: s.display(eU)
f*(a/\b) = (-2*x^2*y^3 - x^3 - (4*x^3 + x)*y^2 - 2*(x^4 + x^2)*y) d/dx/\d/dy
sage: s.display(eV)
f*(a/\b) = (1/2*u^5 - 1/2*u^3*v^2 - 1/2*u^2*v^3 + u^3 + 1/2*(u^4 + 2*u^2)*v)
d/du/\d/dv

bracket(other)

Return the Schouten-Nijenhuis bracket of self with another multivector field.

The Schouten-Nijenhuis bracket extends the Lie bracket of vector fields (cf. bracket()) to multivector fields.

Denoting by $$A^p(M)$$ the $$C^k(M)$$-module of $$p$$-vector fields on the $$C^k$$-differentiable manifold $$M$$ over the field $$K$$ (cf. MultivectorModule), the Schouten-Nijenhuis bracket is a $$K$$-bilinear map

$\begin{split}\begin{array}{ccc} A^p(M)\times A^q(M) & \longrightarrow & A^{p+q-1}(M) \\ (a,b) & \longmapsto & [a,b] \end{array}\end{split}$

which obeys the following properties:

• if $$p=0$$ and $$q=0$$, (i.e. $$a$$ and $$b$$ are two scalar fields), $$[a,b]=0$$
• if $$p=0$$ (i.e. $$a$$ is a scalar field) and $$q\geq 1$$, $$[a,b] = - \iota_{\mathrm{d}a} b$$ (minus the interior product of the differential of $$a$$ by $$b$$)
• if $$p=1$$ (i.e. $$a$$ is a vector field), $$[a,b] = \mathcal{L}_a b$$ (the Lie derivative of $$b$$ along $$a$$)
• $$[a,b] = -(-1)^{(p-1)(q-1)} [b,a]$$
• for any multivector field $$c$$ and $$(a,b) \in A^p(M)\times A^q(M)$$, $$[a,.]$$ obeys the graded Leibniz rule
$[a,b\wedge c] = [a,b]\wedge c + (-1)^{(p-1)q} b\wedge [a,c]$
• for $$(a,b,c) \in A^p(M)\times A^q(M)\times A^r(M)$$, the graded Jacobi identity holds:
$(-1)^{(p-1)(r-1)}[a,[b,c]] + (-1)^{(q-1)(p-1)}[b,[c,a]] + (-1)^{(r-1)(q-1)}[c,[a,b]] = 0$

Note

There are two definitions of the Schouten-Nijenhuis bracket in the literature, which differ from each other when $$p$$ is even by an overall sign. The definition adopted here is that of [Mar1997], [Kos1985] and Wikipedia article Schouten-Nijenhuis_bracket. The other definition, adopted e.g. by [Nij1955], [Lic1977] and [Vai1994], is $$[a,b]' = (-1)^{p+1} [a,b]$$.

INPUT:

• other – a multivector field

OUTPUT:

EXAMPLES:

Bracket of two vector fields on the 2-sphere:

sage: M = Manifold(2, 'S^2', start_index=1) # the sphere S^2
sage: U = M.open_subset('U') ; V = M.open_subset('V')
sage: M.declare_union(U,V)   # S^2 is the union of U and V
sage: c_xy.<x,y> = U.chart() # stereographic coord. North
sage: c_uv.<u,v> = V.chart() # stereographic coord. South
sage: xy_to_uv = c_xy.transition_map(c_uv, (x/(x^2+y^2), y/(x^2+y^2)),
....:                intersection_name='W', restrictions1= x^2+y^2!=0,
....:                restrictions2= u^2+v^2!=0)
sage: uv_to_xy = xy_to_uv.inverse()
sage: W = U.intersection(V) # The complement of the two poles
sage: e_xy = c_xy.frame() ; e_uv = c_uv.frame()
sage: a = M.vector_field({e_xy: [y, x]}, name='a')
sage: b = M.vector_field({e_xy: [x*y, x-y]}, name='b')
sage: s = a.bracket(b); s
Vector field [a,b] on the 2-dimensional differentiable manifold S^2
sage: s.display(e_xy)
[a,b] = (x^2 + y^2 - x + y) d/dx + (-(x - 1)*y - x) d/dy


For two vector fields, the bracket coincides with the Lie derivative:

sage: s == b.lie_derivative(a)
True


Schouten-Nijenhuis bracket of a 2-vector field and a 1-vector field:

sage: c = a.wedge(b); c
2-vector field a/\b on the 2-dimensional differentiable
manifold S^2
sage: s = c.bracket(a); s
2-vector field [a/\b,a] on the 2-dimensional differentiable
manifold S^2
sage: s.display(e_xy)
[a/\b,a] = (x^3 + (2*x - 1)*y^2 - x^2 + 2*x*y) d/dx/\d/dy


Since $$a$$ is a vector field, we have in this case:

sage: s == - c.lie_derivative(a)
True


MultivectorFieldParal.bracket() for more examples and check of standards identities involving the Schouten-Nijenhuis bracket

degree()

Return the degree of self.

OUTPUT:

• integer $$p$$ such that self is a $$p$$-vector field

EXAMPLES:

sage: M = Manifold(3, 'M')
sage: a = M.multivector_field(2); a
2-vector field on the 3-dimensional differentiable manifold M
sage: a.degree()
2
sage: b = M.vector_field(); b
Vector field on the 3-dimensional differentiable manifold M
sage: b.degree()
1

interior_product(form)

Interior product with a differential form.

If self is a multivector field $$A$$ of degree $$p$$ and $$B$$ is a differential form of degree $$q\geq p$$ on the same manifold as $$A$$, the interior product of $$A$$ by $$B$$ is the differential form $$\iota_A B$$ of degree $$q-p$$ defined by

$(\iota_A B)_{i_1\ldots i_{q-p}} = A^{k_1\ldots k_p} B_{k_1\ldots k_p i_1\ldots i_{q-p}}$

Note

A.interior_product(B) yields the same result as A.contract(0,..., p-1, B, 0,..., p-1) (cf. contract()), but interior_product is more efficient, the alternating character of $$A$$ being not used to reduce the computation in contract()

INPUT:

• form – differential form $$B$$ (instance of DiffForm); the degree of $$B$$ must be at least equal to the degree of self

OUTPUT:

• scalar field (case $$p=q$$) or DiffForm (case $$p<q$$) representing the interior product $$\iota_A B$$, where $$A$$ is self

interior_product() for the interior product of a differential form with a multivector field

EXAMPLES:

Interior product of a vector field ($$p=1$$) with a 2-form ($$q=2$$) on the 2-sphere:

sage: M = Manifold(2, 'S^2', start_index=1) # the sphere S^2
sage: U = M.open_subset('U') ; V = M.open_subset('V')
sage: M.declare_union(U,V)   # S^2 is the union of U and V
sage: c_xy.<x,y> = U.chart() # stereographic coord. North
sage: c_uv.<u,v> = V.chart() # stereographic coord. South
sage: xy_to_uv = c_xy.transition_map(c_uv, (x/(x^2+y^2), y/(x^2+y^2)),
....:                intersection_name='W', restrictions1= x^2+y^2!=0,
....:                restrictions2= u^2+v^2!=0)
sage: uv_to_xy = xy_to_uv.inverse()
sage: W = U.intersection(V) # The complement of the two poles
sage: e_xy = c_xy.frame() ; e_uv = c_uv.frame()
sage: a = M.vector_field({e_xy: [-y, x]}, name='a')
sage: b = M.diff_form(2, name='b')
sage: b[e_xy,1,2] = 4/(x^2+y^2+1)^2   # the standard area 2-form
sage: b.display(e_xy)
b = 4/(x^2 + y^2 + 1)^2 dx/\dy
sage: b.display(e_uv)
b = -4/(u^4 + v^4 + 2*(u^2 + 1)*v^2 + 2*u^2 + 1) du/\dv
sage: s = a.interior_product(b); s
1-form i_a b on the 2-dimensional differentiable manifold S^2
sage: s.display(e_xy)
i_a b = -4*x/(x^4 + y^4 + 2*(x^2 + 1)*y^2 + 2*x^2 + 1) dx
- 4*y/(x^4 + y^4 + 2*(x^2 + 1)*y^2 + 2*x^2 + 1) dy
sage: s.display(e_uv)
i_a b = 4*u/(u^4 + v^4 + 2*(u^2 + 1)*v^2 + 2*u^2 + 1) du
+ 4*v/(u^4 + v^4 + 2*(u^2 + 1)*v^2 + 2*u^2 + 1) dv
sage: s == a.contract(b)
True


Example with $$p=2$$ and $$q=2$$:

sage: a = M.multivector_field(2, name='a')
sage: a[e_xy,1,2] = x*y
sage: a.display(e_xy)
a = x*y d/dx/\d/dy
sage: a.display(e_uv)
a = -u*v d/du/\d/dv
sage: s = a.interior_product(b); s
Scalar field i_a b on the 2-dimensional differentiable manifold S^2
sage: s.display()
i_a b: S^2 --> R
on U: (x, y) |--> 8*x*y/(x^4 + y^4 + 2*(x^2 + 1)*y^2 + 2*x^2 + 1)
on V: (u, v) |--> 8*u*v/(u^4 + v^4 + 2*(u^2 + 1)*v^2 + 2*u^2 + 1)


Some checks:

sage: s == a.contract(0, 1, b, 0, 1)
True
sage: s.restrict(U) == 2 * a[[e_xy,1,2]] * b[[e_xy,1,2]]
True
sage: s.restrict(V) == 2 * a[[e_uv,1,2]] * b[[e_uv,1,2]]
True

wedge(other)

Exterior product with another multivector field.

INPUT:

• other – another multivector field (on the same manifold)

OUTPUT:

EXAMPLES:

Exterior product of two vector fields on the 2-sphere:

sage: M = Manifold(2, 'S^2', start_index=1) # the sphere S^2
sage: U = M.open_subset('U') ; V = M.open_subset('V')
sage: M.declare_union(U,V)   # S^2 is the union of U and V
sage: c_xy.<x,y> = U.chart() # stereographic coord. North
sage: c_uv.<u,v> = V.chart() # stereographic coord. South
sage: xy_to_uv = c_xy.transition_map(c_uv, (x/(x^2+y^2), y/(x^2+y^2)),
....:                intersection_name='W', restrictions1= x^2+y^2!=0,
....:                restrictions2= u^2+v^2!=0)
sage: uv_to_xy = xy_to_uv.inverse()
sage: W = U.intersection(V) # The complement of the two poles
sage: e_xy = c_xy.frame() ; e_uv = c_uv.frame()
sage: a = M.vector_field({e_xy: [y, x]}, name='a')
sage: b = M.vector_field({e_xy: [x^2 + y^2, y]}, name='b')
sage: c = a.wedge(b); c
2-vector field a/\b on the 2-dimensional differentiable
manifold S^2
sage: c.display(e_xy)
a/\b = (-x^3 - (x - 1)*y^2) d/dx/\d/dy
sage: c.display(e_uv)
a/\b = (-v^2 + u) d/du/\d/dv

class sage.manifolds.differentiable.multivectorfield.MultivectorFieldParal(vector_field_module, degree, name=None, latex_name=None)

Multivector field with values on a parallelizable manifold.

Given a differentiable manifold $$U$$, a differentiable map $$\Phi: U \rightarrow M$$ to a parallelizable manifold $$M$$ and a positive integer $$p$$, a multivector field of degree $$p$$ (or $$p$$-vector field) along $$U$$ with values on $$M\supset\Phi(U)$$ is a differentiable map

$a:\ U \longrightarrow T^{(p,0)}M$

($$T^{(p,0)}M$$ being the tensor bundle of type $$(p,0)$$ over $$M$$) such that

$\forall x \in U,\quad a(x) \in \Lambda^p(T_{\Phi(x)} M) ,$

where $$T_{\Phi(x)} M$$ is the vector space tangent to $$M$$ at $$\Phi(x)$$ and $$\Lambda^p$$ stands for the exterior power of degree $$p$$ (cf. ExtPowerFreeModule). In other words, $$a(x)$$ is an alternating contravariant tensor of degree $$p$$ of the tangent vector space $$T_{\Phi(x)} M$$.

The standard case of a multivector field on a manifold $$M$$ corresponds to $$U = M$$ and $$\Phi = \mathrm{Id}_M$$. Other common cases are $$\Phi$$ being an immersion and $$\Phi$$ being a curve in $$M$$ ($$U$$ is then an open interval of $$\RR$$).

Note

If $$M$$ is not parallelizable, the class MultivectorField must be used instead.

INPUT:

• vector_field_module – free module $$\mathfrak{X}(U,\Phi)$$ of vector fields along $$U$$ with values on $$M$$ via the map $$\Phi$$
• degree – the degree of the multivector field (i.e. its tensor rank)
• name – (default: None) name given to the multivector field
• latex_name – (default: None) LaTeX symbol to denote the multivector field; if none is provided, the LaTeX symbol is set to name

EXAMPLES:

A 2-vector field on a 4-dimensional manifold:

sage: M = Manifold(4, 'M')
sage: c_txyz.<t,x,y,z> = M.chart()
sage: a = M.multivector_field(2, name='a') ; a
2-vector field a on the 4-dimensional differentiable manifold M
sage: a.parent()
Free module A^2(M) of 2-vector fields on the 4-dimensional
differentiable manifold M


A multivector field is a tensor field of purely contravariant type:

sage: a.tensor_type()
(2, 0)


It is antisymmetric, its components being CompFullyAntiSym:

sage: a.symmetries()
no symmetry;  antisymmetry: (0, 1)
sage: a[0,1] = 2*x
sage: a[1,0]
-2*x
sage: a.comp()
Fully antisymmetric 2-indices components w.r.t. Coordinate frame
(M, (d/dt,d/dx,d/dy,d/dz))
sage: type(a.comp())
<class 'sage.tensor.modules.comp.CompFullyAntiSym'>


Setting a component with repeated indices to a non-zero value results in an error:

sage: a[1,1] = 3
Traceback (most recent call last):
...
ValueError: by antisymmetry, the component cannot have a nonzero value
for the indices (1, 1)
sage: a[1,1] = 0  # OK, albeit useless
sage: a[1,2] = 3  # OK


The expansion of a multivector field with respect to a given frame is displayed via the method display():

sage: a.display() # expansion w.r.t. the default frame
a = 2*x d/dt/\d/dx + 3 d/dx/\d/dy
sage: latex(a.display()) # output for the notebook
a = 2 \, x \frac{\partial}{\partial t }\wedge \frac{\partial}{\partial x }
+ 3 \frac{\partial}{\partial x }\wedge \frac{\partial}{\partial y }


Multivector fields can be added or subtracted:

sage: b = M.multivector_field(2)
sage: b[0,1], b[0,2], b[0,3] = y, 2, x+z
sage: s = a + b ; s
2-vector field on the 4-dimensional differentiable manifold M
sage: s.display()
(2*x + y) d/dt/\d/dx + 2 d/dt/\d/dy + (x + z) d/dt/\d/dz + 3 d/dx/\d/dy
sage: s = a - b ; s
2-vector field on the 4-dimensional differentiable manifold M
sage: s.display()
(2*x - y) d/dt/\d/dx - 2 d/dt/\d/dy + (-x - z) d/dt/\d/dz + 3 d/dx/\d/dy


An example of 3-vector field in $$\RR^3$$ with Cartesian coordinates:

sage: M = Manifold(3, 'R3', latex_name=r'\RR^3', start_index=1)
sage: c_cart.<x,y,z> = M.chart()
sage: a = M.multivector_field(3, name='a')
sage: a[1,2,3] = x^2+y^2+z^2  # the only independent component
sage: a[:] # all the components are set from the previous line:
[[[0, 0, 0], [0, 0, x^2 + y^2 + z^2], [0, -x^2 - y^2 - z^2, 0]],
[[0, 0, -x^2 - y^2 - z^2], [0, 0, 0], [x^2 + y^2 + z^2, 0, 0]],
[[0, x^2 + y^2 + z^2, 0], [-x^2 - y^2 - z^2, 0, 0], [0, 0, 0]]]
sage: a.display()
a = (x^2 + y^2 + z^2) d/dx/\d/dy/\d/dz


Spherical components from the tensorial change-of-frame formula:

sage: c_spher.<r,th,ph> = M.chart(r'r:[0,+oo) th:[0,pi]:\theta ph:[0,2*pi):\phi')
sage: spher_to_cart = c_spher.transition_map(c_cart,
....:                [r*sin(th)*cos(ph), r*sin(th)*sin(ph), r*cos(th)])
sage: cart_to_spher = spher_to_cart.set_inverse(sqrt(x^2+y^2+z^2),
....:                              atan2(sqrt(x^2+y^2),z), atan2(y, x))
Check of the inverse coordinate transformation:
r == r  *passed*
th == arctan2(r*sin(th), r*cos(th))  **failed**
ph == arctan2(r*sin(ph)*sin(th), r*cos(ph)*sin(th))  **failed**
x == x  *passed*
y == y  *passed*
z == z  *passed*
NB: a failed report can reflect a mere lack of simplification.
sage: a.comp(c_spher.frame()) # computation of components w.r.t. spherical frame
Fully antisymmetric 3-indices components w.r.t. Coordinate frame
(R3, (d/dr,d/dth,d/dph))
sage: a.comp(c_spher.frame())[1,2,3, c_spher]
1/sin(th)
sage: a.display(c_spher.frame())
a = sqrt(x^2 + y^2 + z^2)/sqrt(x^2 + y^2) d/dr/\d/dth/\d/dph
sage: a.display(c_spher.frame(), c_spher)
a = 1/sin(th) d/dr/\d/dth/\d/dph


As a shortcut of the above command, on can pass just the chart c_spher to display, the vector frame being then assumed to be the coordinate frame associated with the chart:

sage: a.display(c_spher)
a = 1/sin(th) d/dr/\d/dth/\d/dph


The exterior product of two multivector fields is performed via the method wedge():

sage: a = M.vector_field([x*y, -z*x, y], name='A')
sage: b = M.vector_field([y, z+y, x^2-z^2], name='B')
sage: ab = a.wedge(b) ; ab
2-vector field A/\B on the 3-dimensional differentiable manifold R3
sage: ab.display()
A/\B = (x*y^2 + 2*x*y*z) d/dx/\d/dy + (x^3*y - x*y*z^2 - y^2) d/dx/\d/dz
+ (x*z^3 - y^2 - (x^3 + y)*z) d/dy/\d/dz


Let us check the formula relating the exterior product to the tensor product for vector fields:

sage: a.wedge(b) == a*b - b*a
True


The tensor product of a vector field and a 2-vector field is not a 3-vector field but a tensor field of type $$(3,0)$$ with less symmetries:

sage: c = a*ab ; c
Tensor field A*(A/\B) of type (3,0) on the 3-dimensional differentiable
manifold R3
sage: c.symmetries()  # the antisymmetry is only w.r.t. the last 2 arguments:
no symmetry;  antisymmetry: (1, 2)


The Lie derivative of a 2-vector field is a 2-vector field:

sage: ab.lie_der(a)
2-vector field on the 3-dimensional differentiable manifold R3

bracket(other)

Return the Schouten-Nijenhuis bracket of self with another multivector field.

The Schouten-Nijenhuis bracket extends the Lie bracket of vector fields (cf. bracket()) to multivector fields.

Denoting by $$A^p(M)$$ the $$C^k(M)$$-module of $$p$$-vector fields on the $$C^k$$-differentiable manifold $$M$$ over the field $$K$$ (cf. MultivectorModule), the Schouten-Nijenhuis bracket is a $$K$$-bilinear map

$\begin{split}\begin{array}{ccc} A^p(M)\times A^q(M) & \longrightarrow & A^{p+q-1}(M) \\ (a,b) & \longmapsto & [a,b] \end{array}\end{split}$

which obeys the following properties:

• if $$p=0$$ and $$q=0$$, (i.e. $$a$$ and $$b$$ are two scalar fields), $$[a,b]=0$$
• if $$p=0$$ (i.e. $$a$$ is a scalar field) and $$q\geq 1$$, $$[a,b] = - \iota_{\mathrm{d}a} b$$ (minus the interior product of the differential of $$a$$ by $$b$$)
• if $$p=1$$ (i.e. $$a$$ is a vector field), $$[a,b] = \mathcal{L}_a b$$ (the Lie derivative of $$b$$ along $$a$$)
• $$[a,b] = -(-1)^{(p-1)(q-1)} [b,a]$$
• for any multivector field $$c$$ and $$(a,b) \in A^p(M)\times A^q(M)$$, $$[a,.]$$ obeys the graded Leibniz rule
$[a,b\wedge c] = [a,b]\wedge c + (-1)^{(p-1)q} b\wedge [a,c]$
• for $$(a,b,c) \in A^p(M)\times A^q(M)\times A^r(M)$$, the graded Jacobi identity holds:
$(-1)^{(p-1)(r-1)}[a,[b,c]] + (-1)^{(q-1)(p-1)}[b,[c,a]] + (-1)^{(r-1)(q-1)}[c,[a,b]] = 0$

Note

There are two definitions of the Schouten-Nijenhuis bracket in the literature, which differ from each other when $$p$$ is even by an overall sign. The definition adopted here is that of [Mar1997], [Kos1985] and Wikipedia article Schouten-Nijenhuis_bracket. The other definition, adopted e.g. by [Nij1955], [Lic1977] and [Vai1994], is $$[a,b]' = (-1)^{p+1} [a,b]$$.

INPUT:

• other – a multivector field

OUTPUT:

EXAMPLES:

Let us consider two vector fields on a 3-dimensional manifold:

sage: M = Manifold(3, 'M')
sage: X.<x,y,z> = M.chart()
sage: a = M.vector_field([x*y+z, x+y-z, z-2*x+y], name='a')
sage: b = M.vector_field([y+2*z-x, x^2-y+z, z-x], name='b')


and form their Schouten-Nijenhuis bracket:

sage: s = a.bracket(b); s
Vector field [a,b] on the 3-dimensional differentiable manifold M
sage: s.display()
[a,b] = (-x^3 + (x + 3)*y - y^2 - (x + 2*y + 1)*z - 2*x) d/dx
+ (2*x^2*y - x^2 + 2*x*z - 3*x) d/dy
+ (-x^2 - (x - 4)*y - 3*x + 2*z) d/dz


Check that $$[a,b]$$ is actually the Lie bracket:

sage: f = M.scalar_field({X: x+y*z}, name='f')
sage: s(f) == a(b(f)) - b(a(f))
True


Check that $$[a,b]$$ coincides with the Lie derivative of $$b$$ along $$a$$:

sage: s == b.lie_derivative(a)
True


Schouten-Nijenhuis bracket for $$p=0$$ and $$q=1$$:

sage: s = f.bracket(a); s
Scalar field -i_df a on the 3-dimensional differentiable manifold M
sage: s.display()
-i_df a: M --> R
(x, y, z) |--> x*y - y^2 - (x + 2*y + 1)*z + z^2


Check that $$[f,a] = - \iota_{\mathrm{d}f} a = - \mathrm{d}f(a)$$:

sage: s == - f.differential()(a)
True


Schouten-Nijenhuis bracket for $$p=0$$ and $$q=2$$:

sage: c = M.multivector_field(2, name='c')
sage: c[0,1], c[0,2], c[1,2] = x+z+1, x*y+z, x-y
sage: s = f.bracket(c); s
Vector field -i_df c on the 3-dimensional differentiable manifold M
sage: s.display()
-i_df c = (x*y^2 + (x + y + 1)*z + z^2) d/dx
+ (x*y - y^2 - x - z - 1) d/dy + (-x*y - (x - y + 1)*z) d/dz


Check that $$[f,c] = - \iota_{\mathrm{d}f} c$$:

sage: s == - f.differential().interior_product(c)
True


Schouten-Nijenhuis bracket for $$p=1$$ and $$q=2$$:

sage: s = a.bracket(c); s
2-vector field [a,c] on the 3-dimensional differentiable manifold M
sage: s.display()
[a,c] = ((x - 1)*y - (y - 2)*z - 2*x - 1) d/dx/\d/dy
+ ((x + 1)*y - (x + 1)*z - 3*x - 1) d/dx/\d/dz
+ (-5*x + y - z - 2) d/dy/\d/dz


Again, since $$a$$ is a vector field, the Schouten-Nijenhuis bracket coincides with the Lie derivative:

sage: s == c.lie_derivative(a)
True


Schouten-Nijenhuis bracket for $$p=2$$ and $$q=2$$:

sage: d = M.multivector_field(2, name='d')
sage: d[0,1], d[0,2], d[1,2] = x-y^2, x+z, z-x-1
sage: s = c.bracket(d); s
3-vector field [c,d] on the 3-dimensional differentiable manifold M
sage: s.display()
[c,d] = (-y^3 + (3*x + 1)*y - y^2 - x - z + 2) d/dx/\d/dy/\d/dz


Let us check the component formula (with respect to the manifold’s default coordinate chart, i.e. X) for $$p=q=2$$, taking into account the tensor antisymmetries:

sage: s[0,1,2] == - sum(c[i,0]*d[1,2].diff(i)
....:                 + c[i,1]*d[2,0].diff(i) + c[i,2]*d[0,1].diff(i)
....:                 + d[i,0]*c[1,2].diff(i) + d[i,1]*c[2,0].diff(i)
....:                 + d[i,2]*c[0,1].diff(i) for i in M.irange())
True


Schouten-Nijenhuis bracket for $$p=1$$ and $$q=3$$:

sage: e = M.multivector_field(3, name='e')
sage: e[0,1,2] = x+y*z+1
sage: s = a.bracket(e); s
3-vector field [a,e] on the 3-dimensional differentiable manifold M
sage: s.display()
[a,e] = (-(2*x + 1)*y + y^2 - (y^2 - x - 1)*z - z^2
- 2*x - 2) d/dx/\d/dy/\d/dz


Again, since $$p=1$$, the bracket coincides with the Lie derivative:

sage: s == e.lie_derivative(a)
True


Schouten-Nijenhuis bracket for $$p=2$$ and $$q=3$$:

sage: s = c.bracket(e); s
4-vector field [c,e] on the 3-dimensional differentiable manifold M


Since on a 3-dimensional manifold, any 4-vector field is zero, we have:

sage: s.display()
[c,e] = 0


Let us check the graded commutation law $$[a,b] = -(-1)^{(p-1)(q-1)} [b,a]$$ for various values of $$p$$ and $$q$$:

sage: f.bracket(a) == - a.bracket(f)  # p=0 and q=1
True
sage: f.bracket(c) == c.bracket(f)    # p=0 and q=2
True
sage: a.bracket(b) == - b.bracket(a)  # p=1 and q=1
True
sage: a.bracket(c) == - c.bracket(a)  # p=1 and q=2
True
sage: c.bracket(d) == d.bracket(c)    # p=2 and q=2
True


Let us check the graded Leibniz rule for $$p=1$$ and $$q=1$$:

sage: a.bracket(b.wedge(c)) == a.bracket(b).wedge(c) + b.wedge(a.bracket(c))
True


as well as for $$p=2$$ and $$q=1$$:

sage: c.bracket(a.wedge(b)) == c.bracket(a).wedge(b) - a.wedge(c.bracket(b))
True


Finally let us check the graded Jacobi identity for $$p=1$$, $$q=1$$ and $$r=2$$:

sage: a.bracket(b.bracket(c)) + b.bracket(c.bracket(a)) \
....: + c.bracket(a.bracket(b)) == 0
True


as well as for $$p=1$$, $$q=2$$ and $$r=2$$:

sage: a.bracket(c.bracket(d)) + c.bracket(d.bracket(a)) \
....: - d.bracket(a.bracket(c)) == 0
True

interior_product(form)

Interior product with a differential form.

If self is a multivector field $$A$$ of degree $$p$$ and $$B$$ is a differential form of degree $$q\geq p$$ on the same manifold as $$A$$, the interior product of $$A$$ by $$B$$ is the differential form $$\iota_A B$$ of degree $$q-p$$ defined by

$(\iota_A B)_{i_1\ldots i_{q-p}} = A^{k_1\ldots k_p} B_{k_1\ldots k_p i_1\ldots i_{q-p}}$

Note

A.interior_product(B) yields the same result as A.contract(0,..., p-1, B, 0,..., p-1) (cf. contract()), but interior_product is more efficient, the alternating character of $$A$$ being not used to reduce the computation in contract()

INPUT:

• form – differential form $$B$$ (instance of DiffFormParal); the degree of $$B$$ must be at least equal to the degree of self

OUTPUT:

• scalar field (case $$p=q$$) or DiffFormParal (case $$p<q$$) representing the interior product $$\iota_A B$$, where $$A$$ is self

interior_product() for the interior product of a differential form with a multivector field

EXAMPLES:

Interior product with $$p=1$$ and $$q=1$$ on 4-dimensional manifold:

sage: M = Manifold(4, 'M')
sage: X.<t,x,y,z> = M.chart()
sage: a = M.vector_field([x, 1+t^2, x*z, y-3], name='a')
sage: b = M.one_form([-z^2, 2, x, x-y], name='b')
sage: s = a.interior_product(b); s
Scalar field i_a b on the 4-dimensional differentiable manifold M
sage: s.display()
i_a b: M --> R
(t, x, y, z) |--> x^2*z - x*z^2 + 2*t^2 + (x + 3)*y - y^2
- 3*x + 2


In this case, we have $$\iota_a b = a^i b_i = a(b) = b(a)$$:

sage: all([s == a.contract(b), s == a(b), s == b(a)])
True


Case $$p=1$$ and $$q=3$$:

sage: c = M.diff_form(3, name='c')
sage: c[0,1,2], c[0,1,3] = x*y - z, -3*t
sage: c[0,2,3], c[1,2,3] = t+x, y
sage: s = a.interior_product(c); s
2-form i_a c on the 4-dimensional differentiable manifold M
sage: s.display()
i_a c = (x^2*y*z - x*z^2 - 3*t*y + 9*t) dt/\dx
+ (-(t^2*x - t)*y + (t^2 + 1)*z - 3*t - 3*x) dt/\dy
+ (3*t^3 - (t*x + x^2)*z + 3*t) dt/\dz
+ ((x^2 - 3)*y + y^2 - x*z) dx/\dy
+ (-x*y*z - 3*t*x) dx/\dz + (t*x + x^2 + (t^2 + 1)*y) dy/\dz
sage: s == a.contract(c)
True


Case $$p=2$$ and $$q=3$$:

sage: d = M.multivector_field(2, name='d')
sage: d[0,1], d[0,2], d[0,3] = t-x, 2*z, y-1
sage: d[1,2], d[1,3], d[2,3] = z, y+t, 4
sage: s = d.interior_product(c); s
1-form i_d c on the 4-dimensional differentiable manifold M
sage: s.display()
i_d c = (2*x*y*z - 6*t^2 - 6*t*y - 2*z^2 + 8*t + 8*x) dt
+ (-4*x*y*z + 2*(3*t + 4)*y + 4*z^2 - 6*t) dx
+ (2*((t - 1)*x - x^2 - 2*t)*y - 2*y^2 - 2*(t - x)*z + 2*t
+ 2*x) dy + (-6*t^2 + 6*t*x + 2*(2*t + 2*x + y)*z) dz
sage: s == d.contract(0, 1, c, 0, 1)
True

wedge(other)

Exterior product of self with another multivector field.

INPUT:

• other – another multivector field

OUTPUT:

EXAMPLES:

Exterior product of a vector field and a 2-vector field on a 3-dimensional manifold:

sage: M = Manifold(3, 'M', start_index=1)
sage: X.<x,y,z> = M.chart()
sage: a = M.vector_field([2, 1+x, y*z], name='a')
sage: b = M.multivector_field(2, name='b')
sage: b[1,2], b[1,3], b[2,3] = y^2, z+x, z^2
sage: a.display()
a = 2 d/dx + (x + 1) d/dy + y*z d/dz
sage: b.display()
b = y^2 d/dx/\d/dy + (x + z) d/dx/\d/dz + z^2 d/dy/\d/dz
sage: s = a.wedge(b); s
3-vector field a/\b on the 3-dimensional differentiable
manifold M
sage: s.display()
a/\b = (-x^2 + (y^3 - x - 1)*z + 2*z^2 - x) d/dx/\d/dy/\d/dz


Check:

sage: s[1,2,3] == a*b[2,3] + a*b[3,1] + a*b[1,2]
True