# Miscellaneous Functions¶

This file contains several miscellaneous functions used by $$p$$-adics.

• gauss_sum – compute Gauss sums using the Gross-Koblitz formula.

• min – a version of min that returns $$\infty$$ on empty input.

• max – a version of max that returns $$-\infty$$ on empty input.

AUTHORS:

• David Roe

• Ander Steele

• Kiran Kedlaya (modified gauss_sum 2017/09)

sage.rings.padics.misc.gauss_sum(a, p, f, prec=20, factored=False, algorithm='pari', parent=None)

Return the Gauss sum $$g_q(a)$$ as a $$p$$-adic number.

The Gauss sum $$g_q(a)$$ is defined by

$g_q(a)= \sum_{u\in F_q^*} \omega(u)^{-a} \zeta_q^u,$

where $$q = p^f$$, $$\omega$$ is the Teichmüller character and $$\zeta_q$$ is some arbitrary choice of primitive $$q$$-th root of unity. The computation is adapted from the main theorem in Alain Robert’s paper The Gross-Koblitz formula revisited, Rend. Sem. Mat. Univ. Padova 105 (2001), 157–170.

Let $$p$$ be a prime, $$f$$ a positive integer, $$q=p^f$$, and $$\pi$$ be the unique root of $$f(x) = x^{p-1}+p$$ congruent to $$\zeta_p - 1$$ modulo $$(\zeta_p - 1)^2$$. Let $$0\leq a < q-1$$. Then the Gross-Koblitz formula gives us the value of the Gauss sum $$g_q(a)$$ as a product of $$p$$-adic Gamma functions as follows:

$g_q(a) = -\pi^s \prod_{0\leq i < f} \Gamma_p(a^{(i)}/(q-1)),$

where $$s$$ is the sum of the digits of $$a$$ in base $$p$$ and the $$a^{(i)}$$ have $$p$$-adic expansions obtained from cyclic permutations of that of $$a$$.

INPUT:

• a – integer

• p – prime

• f – positive integer

• prec – positive integer (optional, 20 by default)

• factored - boolean (optional, False by default)

• algorithm - flag passed to p-adic Gamma function (optional, “pari” by default)

OUTPUT:

If factored is False, returns a $$p$$-adic number in an Eisenstein extension of $$\QQ_p$$. This number has the form $$pi^e * z$$ where $$pi$$ is as above, $$e$$ is some nonnegative integer, and $$z$$ is an element of $$\ZZ_p$$; if factored is True, the pair $$(e,z)$$ is returned instead, and the Eisenstein extension is not formed.

Note

This is based on GP code written by Adriana Salerno.

EXAMPLES:

In this example, we verify that $$g_3(0) = -1$$:

sage: from sage.rings.padics.misc import gauss_sum
sage: -gauss_sum(0,3,1)
1 + O(pi^40)


Next, we verify that $$g_5(a) g_5(-a) = 5 (-1)^a$$:

sage: from sage.rings.padics.misc import gauss_sum
sage: gauss_sum(2,5,1)^2-5
O(pi^84)
sage: gauss_sum(1,5,1)*gauss_sum(3,5,1)+5
O(pi^84)


Finally, we compute a non-trivial value:

sage: from sage.rings.padics.misc import gauss_sum
sage: gauss_sum(2,13,2)
6*pi^2 + 7*pi^14 + 11*pi^26 + 3*pi^62 + 6*pi^74 + 3*pi^86 + 5*pi^98 +
pi^110 + 7*pi^134 + 9*pi^146 + 4*pi^158 + 6*pi^170 + 4*pi^194 +
pi^206 + 6*pi^218 + 9*pi^230 + O(pi^242)
sage: gauss_sum(2,13,2,prec=5,factored=True)
(2, 6 + 6*13 + 10*13^2 + O(13^5))


Return the maximum of the inputs, where the maximum of the empty list is $$-\infty$$.

EXAMPLES:

sage: from sage.rings.padics.misc import max
sage: max()
-Infinity
sage: max(2,3)
3


Return the minimum of the inputs, where the minimum of the empty list is $$\infty$$.

EXAMPLES:

sage: from sage.rings.padics.misc import min
sage: min()
+Infinity
sage: min(2,3)
2


String describing the precision mode on a p-adic ring or field.

EXAMPLES:

sage: from sage.rings.padics.misc import precprint
sage: precprint('capped-rel', 12, 2)
'with capped relative precision 12'
sage: precprint('capped-abs', 11, 3)
'with capped absolute precision 11'
sage: precprint('floating-point', 1234, 5)
'with floating precision 1234'
sage: precprint('fixed-mod', 1, 17)
'of fixed modulus 17^1'


Strips trailing zeros/empty lists from a list.

EXAMPLES:

sage: from sage.rings.padics.misc import trim_zeros
sage: trim_zeros([1,0,1,0])
[1, 0, 1]
sage: trim_zeros([,[],,[],[]])
[, [], ]
sage: trim_zeros([[],[]])
[]
sage: trim_zeros([])
[]


Zeros are also trimmed from nested lists (one deep):

sage: trim_zeros([[1,0]]) [] sage: trim_zeros([,]) [[], ]