# Helper functions for local components¶

This module contains various functions relating to lifting elements of $$\mathrm{SL}_2(\ZZ / N\ZZ)$$ to $$\mathrm{SL}_2(\ZZ)$$, and other related problems.

sage.modular.local_comp.liftings.lift_for_SL(A, N=None)

Lift a matrix $$A$$ from $$SL_m(\ZZ / N\ZZ)$$ to $$SL_m(\ZZ)$$.

This follows [Shi1971], Lemma 1.38, p. 21.

INPUT:

• A – a square matrix with coefficients in $$\ZZ / N\ZZ$$ (or $$\ZZ$$)
• N – the modulus (optional) required only if the matrix A has coefficients in $$\ZZ$$

EXAMPLES:

sage: from sage.modular.local_comp.liftings import lift_for_SL
sage: A = matrix(Zmod(11), 4, 4, [6, 0, 0, 9, 1, 6, 9, 4, 4, 4, 8, 0, 4, 0, 0, 8])
sage: A.det()
1
sage: L = lift_for_SL(A)
sage: L.det()
1
sage: (L - A) == 0
True

sage: B = matrix(Zmod(19), 4, 4, [1, 6, 10, 4, 4, 14, 15, 4, 13, 0, 1, 15, 15, 15, 17, 10])
sage: B.det()
1
sage: L = lift_for_SL(B)
sage: L.det()
1
sage: (L - B) == 0
True

sage.modular.local_comp.liftings.lift_gen_to_gamma1(m, n)

Return four integers defining a matrix in $$\mathrm{SL}_2(\ZZ)$$ which is congruent to $$\begin{pmatrix} 0 & -1 \\ 1 & 0 \end{pmatrix} \pmod m$$ and lies in the subgroup $$\begin{pmatrix} 1 & * \\ 0 & 1 \end{pmatrix} \pmod n$$.

This is a special case of lift_to_gamma1(), and is coded as such.

INPUT:

• $$m$$, $$n$$ – coprime positive integers

EXAMPLES:

sage: from sage.modular.local_comp.liftings import lift_gen_to_gamma1
sage: A = matrix(ZZ, 2, lift_gen_to_gamma1(9, 8)); A
[441  62]
[ 64   9]
sage: A.change_ring(Zmod(9))
[0 8]
[1 0]
sage: A.change_ring(Zmod(8))
[1 6]
[0 1]
sage: type(lift_gen_to_gamma1(9, 8)[0])
<type 'sage.rings.integer.Integer'>

sage.modular.local_comp.liftings.lift_matrix_to_sl2z(A, N)

Given a list of length 4 representing a 2x2 matrix over $$\ZZ / N\ZZ$$ with determinant 1 (mod $$N$$), lift it to a 2x2 matrix over $$\ZZ$$ with determinant 1.

This is a special case of lift_to_gamma1(), and is coded as such.

INPUT:

• A – list of 4 integers defining a $$2 \times 2$$ matrix
• $$N$$ – positive integer

EXAMPLES:

sage: from sage.modular.local_comp.liftings import lift_matrix_to_sl2z
sage: lift_matrix_to_sl2z([10, 11, 3, 11], 19)
[29, 106, 3, 11]
sage: type(_[0])
<type 'sage.rings.integer.Integer'>
sage: lift_matrix_to_sl2z([2,0,0,1], 5)
Traceback (most recent call last):
...
ValueError: Determinant is 2 mod 5, should be 1

sage.modular.local_comp.liftings.lift_ramified(g, p, u, n)

Given four integers $$a,b,c,d$$ with $$p \mid c$$ and $$ad - bc = 1 \pmod{p^u}$$, find $$a',b',c',d'$$ congruent to $$a,b,c,d \pmod{p^u}$$, with $$c' = c \pmod{p^{u+1}}$$, such that $$a'd' - b'c'$$ is exactly 1, and $$\begin{pmatrix} a & b \\ c & d \end{pmatrix}$$ is in $$\Gamma_1(n)$$.

Algorithm: Uses lift_to_gamma1() to get a lifting modulo $$p^u$$, and then adds an appropriate multiple of the top row to the bottom row in order to get the bottom-left entry correct modulo $$p^{u+1}$$.

EXAMPLES:

sage: from sage.modular.local_comp.liftings import lift_ramified
sage: lift_ramified([2,2,3,2], 3, 1, 1)
[5, 8, 3, 5]
sage: lift_ramified([8,2,12,2], 3, 2, 23)
[323, 110, -133584, -45493]
sage: type(lift_ramified([8,2,12,2], 3, 2, 23)[0])
<type 'sage.rings.integer.Integer'>

sage.modular.local_comp.liftings.lift_to_gamma1(g, m, n)

If g = [a,b,c,d] is a list of integers defining a $$2 \times 2$$ matrix whose determinant is $$1 \pmod m$$, return a list of integers giving the entries of a matrix which is congruent to $$g \pmod m$$ and to $$\begin{pmatrix} 1 & * \\ 0 & 1 \end{pmatrix} \pmod n$$. Here $$m$$ and $$n$$ must be coprime.

INPUT:

• g – list of 4 integers defining a $$2 \times 2$$ matrix
• $$m$$, $$n$$ – coprime positive integers

Here $$m$$ and $$n$$ should be coprime positive integers. Either of $$m$$ and $$n$$ can be $$1$$. If $$n = 1$$, this still makes perfect sense; this is what is called by the function lift_matrix_to_sl2z(). If $$m = 1$$ this is a rather silly question, so we adopt the convention of always returning the identity matrix.

The result is always a list of Sage integers (unlike lift_to_sl2z, which tends to return Python ints).

EXAMPLES:

sage: from sage.modular.local_comp.liftings import lift_to_gamma1
sage: A = matrix(ZZ, 2, lift_to_gamma1([10, 11, 3, 11], 19, 5)); A
[371  68]
[ 60  11]
sage: A.det() == 1
True
sage: A.change_ring(Zmod(19))
[10 11]
[ 3 11]
sage: A.change_ring(Zmod(5))
[1 3]
[0 1]
sage: m = list(SL2Z.random_element())
sage: n = lift_to_gamma1(m, 11, 17)
sage: assert matrix(Zmod(11), 2, n) == matrix(Zmod(11),2,m)
sage: assert matrix(Zmod(17), 2, [n[0], 0, n[2], n[3]]) == 1
sage: type(lift_to_gamma1([10,11,3,11],19,5)[0])
<type 'sage.rings.integer.Integer'>


Tests with $$m = 1$$ and with $$n = 1$$:

sage: lift_to_gamma1([1,1,0,1], 5, 1)
[1, 1, 0, 1]
sage: lift_to_gamma1([2,3,11,22], 1, 5)
[1, 0, 0, 1]

sage.modular.local_comp.liftings.lift_uniformiser_odd(p, u, n)

Construct a matrix over $$\ZZ$$ whose determinant is $$p$$, and which is congruent to $$\begin{pmatrix} 0 & -1 \\ p & 0 \end{pmatrix} \pmod{p^u}$$ and to $$\begin{pmatrix} p & 0 \\ 0 & 1\end{pmatrix} \pmod n$$.

This is required for the local components machinery in the “ramified” case (when the exponent of $$p$$ dividing the level is odd).

EXAMPLES:

sage: from sage.modular.local_comp.liftings import lift_uniformiser_odd
sage: lift_uniformiser_odd(3, 2, 11)
[432, 377, 165, 144]
sage: type(lift_uniformiser_odd(3, 2, 11)[0])
<type 'sage.rings.integer.Integer'>