S2 -- Universal map to a module satisfying Serre's condition S2

Synopsis

Usage:

f = S2(b,M)
Inputs:
- b, an integer, degree bound to which to carry the computation
- M, a module,
Outputs:
- f, a matrix, defining a map M-->M' that agrees with the S2-ification of M in degrees $\geq b$

Description

If M is a graded module over a ring S, then the S2-ification of M is \sum_{d \in ZZ} H^0((sheaf M)(d)), which may be computed as lim_{d->\infty} Hom(I_d,M), where I_d is any sequence of ideals contained in higher and higher powers of S_+. There is a natural restriction map f: M = Hom(S,M) \to Hom(I_d,M). We compute all this using the ideals I_d generated by the d-th powers of the variables in S.

Since the result may not be finitely generated (this happens if and only if M has an associated prime of dimension 1), we compute only up to a specified degree bound b. For the result to be correct down to degree b, it is sufficient to compute Hom(I,M) where I \subset (S_+)^{r-b}.

i1 : kk=ZZ/101

o1 = kk

o1 : QuotientRing

i2 : S = kk[a,b,c,d]

o2 = S

o2 : PolynomialRing

i3 : M = truncate(3,S^1)

o3 = image | d3 cd2 bd2 ad2 c2d bcd acd b2d abd a2d c3 bc2 ac2 b2c abc a2c b3 ab2 a2b a3 |

                             1
o3 : S-module, submodule of S

i4 : betti matrix S2(0,M)

            0  1
o4 = total: 1 20
         0: 1  .
         1: .  .
         2: . 20

o4 : BettiTally

i5 : betti matrix S2(1,M)

            0  1
o5 = total: 1 20
         0: 1  .
         1: .  .
         2: . 20

o5 : BettiTally

i6 : M = S^1/intersect(ideal"a,b,c", ideal"b,c,d",ideal"c,d,a",ideal"d,a,b")

o6 = cokernel | cd bd ad bc ac ab |

                            1
o6 : S-module, quotient of S

i7 : prune source S2(0,M)

o7 = cokernel | cd bd ad bc ac ab |

                            1
o7 : S-module, quotient of S

i8 : prune target S2(0,M)

o8 = cokernel {-1} | d c b 0 0 0 0 0 0 0 0 0 |
              {-1} | 0 0 0 d c a 0 0 0 0 0 0 |
              {-1} | 0 0 0 0 0 0 d b a 0 0 0 |
              {-1} | 0 0 0 0 0 0 0 0 0 c b a |

                            4
o8 : S-module, quotient of S

At one time DE hoped that, if M were a module over the complete intersection R with residue field k, then the natural map from "complete" Ext module "(widehat Ext)_R(M,k)" to the S2-ification of Ext_R(M,k) would be surjective; equivalently, if N were a sufficiently negative syzygy of M, then the first local cohomology module of Ext_R(M,k) would be zero. This is false, as shown by the following example:

i9 : S = ZZ/101[x_0..x_2];

i10 : ff = apply(3, i->x_i^2);

i11 : R = S/ideal ff;

i12 : M = cokernel matrix {{x_0, x_1*x_2}};

i13 : b = 5;

i14 : Mb = prune syzygyModule(-b,M);

i15 : E = prune evenExtModule Mb;

i16 : S2map = S2(0,E);

o16 : Matrix

i17 : SE = prune target S2map;

i18 : extra = prune coker S2map;

i19 : KE = prune ker S2map;

i20 : betti res(Mb, LengthLimit => 10)

              0  1 2 3 4 5 6 7 8  9 10
o20 = total: 20 14 9 5 2 1 2 4 7 11 16
         -6: 20 14 9 5 2 . . . .  .  .
         -5:  .  . . . . 1 1 1 1  1  1
         -4:  .  . . . . . 1 3 6 10 15

o20 : BettiTally

i21 : apply (5, i-> hilbertFunction(i, KE))

o21 = {20, 9, 2, 0, 0}

o21 : List

i22 : apply (5, i-> hilbertFunction(i, E))

o22 = {20, 9, 2, 2, 7}

o22 : List

i23 : apply (5, i-> hilbertFunction(i, SE))

o23 = {1, 1, 1, 2, 7}

o23 : List

i24 : apply (5, i-> hilbertFunction(i, extra))

o24 = {1, 1, 1, 0, 0}

o24 : List

Caveat

Text S2-ification is related to computing cohomology and to computing integral closure; there are scripts in those packages that produce an S2-ification, but one takes a ring as argument and the other doesn't produce the comparison map.

Ways to use S2:

S2(ZZ,Module)

For the programmer

The object S2 is a method function.