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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
• Adriana Salerno
• 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))

See also

• sage.arith.misc.gauss_sum() for general finite fields
• sage.modular.dirichlet.DirichletCharacter.gauss_sum() for prime finite fields
• sage.modular.dirichlet.DirichletCharacter.gauss_sum_numerical() for prime finite fields

sage.rings.padics.misc.max(*L)

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

sage.rings.padics.misc.min(*L)

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

sage.rings.padics.misc.precprint(prec_type, prec_cap, p)

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'

sage.rings.padics.misc.trim_zeros(L)

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([[1],[],[2],[],[]])
[[1], [], [2]]
sage: trim_zeros([[],[]])
[]
sage: trim_zeros([])
[]

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

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