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# golodBetti -- list the ranks of the free modules in the resolution of a Golod module

- Usage:
    `B = golodBetti(M,b)`
- Inputs:
  - M, a module, R-module
  - b, an integer, homological degree to which to carry the computation
- Outputs:
  - B, a Betti tally, This would be betti table of the free res of M over R, if M were a Golod module over R

## Description

Let S be a standard graded polynomial ring. A module M over R = S/I is Golod if the resolution H of M has maximal betti numbers given the betti numbers of the S-free resolutions F of R and K of M. This resolution, H, has underlying graded module H = R**K**T(B), where B is the truncated resolution F_1 <- F_2... and T(B) is the tensor algebra.

Since the component modules of H are given, the computation only requires the computation of the minimal S-free resolution of M, and then is purely numeric; the differentials in the R-free resolution of M are not computed.

In case M = coker vars R, the result is the Betti table of the Golod-Shamash-Eagon resolution of the residue field.

We say that M is a Golod module (over R) if the ranks of the free modules in a minimal R-free resolution of M are equal to the numbers produced by golodBetti. Theorems of Levin and Lescot assert that if R has a Golod module, then R is a Golod ring; and that if R is Golod, then the d-th syzygy of any R-module M is Golod for all d greater than or equal to the projective dimension of M as an S-module (more generally, the co-depth of M) (Avramov, 6 lectures, 5.3.2).

    i1 : S = ZZ/101[a,b,c]

    o1 = S

    o1 : PolynomialRing

    i2 : I = (ideal(a,b,c^2))^2

                 2          2   2     2   4
    o2 = ideal (a , a*b, a*c , b , b*c , c )

    o2 : Ideal of S

    i3 : F = res(S^1/I)

          1      6      8      3
    o3 = S  <-- S  <-- S  <-- S  <-- 0

         0      1      2      3      4

    o3 : ChainComplex

    i4 : R = S/I

    o4 = R

    o4 : QuotientRing

    i5 : F = burkeResolution (coker vars R, 6)

          1      3      9      27      81      243      729
    o5 = R  <-- R  <-- R  <-- R   <-- R   <-- R    <-- R

         0      1      2      3       4       5        6

    o5 : Complex

    i6 : golodBetti(coker vars R,6)

                0 1 2  3  4   5   6
    o6 = total: 1 3 9 27 81 243 729
             0: 1 3 6 12 24  48  96
             1: . . 2 10 32  88 224
             2: . . 1  5 20  72 232
             3: . . .  .  4  28 128
             4: . . .  .  1   7  42
             5: . . .  .  .   .   6
             6: . . .  .  .   .   1

    o6 : BettiTally

    i7 : betti res (coker vars R, LengthLimit => 6)

                0 1 2  3  4   5   6
    o7 = total: 1 3 9 27 81 243 729
             0: 1 3 6 12 24  48  96
             1: . . 2 10 32  88 224
             2: . . 1  5 20  72 232
             3: . . .  .  4  28 128
             4: . . .  .  1   7  42
             5: . . .  .  .   .   6
             6: . . .  .  .   .   1

    o7 : BettiTally

    i8 : betti F

                0 1 2  3  4   5   6
    o8 = total: 1 3 9 27 81 243 729
             0: 1 3 6 12 24  48  96
             1: . . 2 10 32  88 224
             2: . . 1  5 20  72 232
             3: . . .  .  4  28 128
             4: . . .  .  1   7  42
             5: . . .  .  .   .   6
             6: . . .  .  .   .   1

    o8 : BettiTally

## See also

- burkeResolution -- compute a resolution from A-infinity structures

## Ways to use golodBetti:

- `golodBetti(Module,ZZ)`

## For the programmer

The object golodBetti is a method function.

The source of this document is in /build/macaulay2-88fgJW/macaulay2-1.25.11+ds/M2/Macaulay2/packages/AInfinity.m2:1388:0.