# Spectral sequences and connecting morphisms

If $0 \rightarrow A \rightarrow B \rightarrow C \rightarrow 0$ is a short exact sequence of chain complexes then the connecting morphism $H_i(C) \rightarrow H_{i - 1}(A)$ can realized as a suitable map on the $E^1$ of a spectral sequence determined by a suitably defined two step filtration of $B$.

Here we illustrate this realization in a concrete situation: we compute the connecting morphism $H^i(X, F) \rightarrow H^{i + 1}(X, G)$ arising from a short exact sequence $0 \rightarrow G \rightarrow H \rightarrow F \rightarrow 0$ of sheaves on a smooth toric variety $X$.

More specifically we let $X = \mathbb{P}^1 \times \mathbb{P}^1$ and use multigraded commutative algebra together with spectral sequences to compute the connecting morphism $H^1(C, OO_C(1,0)) \rightarrow H^2(X, OO_X(-2,-3))$ where $C$ is a general divisor of type $(3,3)$ on $X$. This connecting morphism is an isomorphism.

 i1 : R = ZZ/101[a_0..b_1, Degrees=>{2:{1,0},2:{0,1}}]; -- PP^1 x PP^1 i2 : M = intersect(ideal(a_0,a_1),ideal(b_0,b_1)) ; -- irrelevant ideal o2 : Ideal of R i3 : M = M_*/(x -> x^5)//ideal ; -- Suitably high Frobenius power of M o3 : Ideal of R i4 : G = res image gens M ; i5 : I = ideal random(R^1, R^{{-3,-3}}) -- ideal of C 3 3 2 3 2 3 3 3 3 2 2 2 o5 = ideal(24a b + 19a a b - 8a a b - 38a b - 36a b b + 19a a b b - 0 0 0 1 0 0 1 0 1 0 0 0 1 0 1 0 1 ------------------------------------------------------------------------ 2 2 3 2 3 2 2 2 2 2 3 2 22a a b b - 16a b b - 30a b b - 10a a b b - 29a a b b + 39a b b - 0 1 0 1 1 0 1 0 0 1 0 1 0 1 0 1 0 1 1 0 1 ------------------------------------------------------------------------ 3 3 2 3 2 3 3 3 29a b - 29a a b - 24a a b + 21a b ) 0 1 0 1 1 0 1 1 1 1 o5 : Ideal of R i6 : b = chainComplex gradedModule R^{{1,0}} -- make line bundle a chain complex 1 o6 = R 0 o6 : ChainComplex i7 : a = chainComplex gradedModule R^{{-2,-3}} 1 o7 = R 0 o7 : ChainComplex i8 : f = chainComplexMap(b, a,{random(R^1, R^{{-3,-3}})}) ; i9 : K = filteredComplex ({Hom(G,f)}) ; -- the two step filtered complex we want i10 : E = prune spectralSequence K ;

The degree zero piece of the map $E^1 .dd_{1, -2}$ below is the desired connecting morphism $H^1(C, OO_C(1,0)) \rightarrow H^2(X, OO_X(-2,-3))$.

 i11 : E^1 .dd_{1,-2} -- the connecting map HH^1(C, OO_C(1,0)) --> HH^2(X, OO_X(-2,-3)) o11 = {-8, -7} | 0 0 -31b_0^4b_1^2-29b_0^3b_1^3+31b_0^2b_1^4 ----------------------------------------------------------------------- -22b_0^4b_1^2-9b_0^3b_1^3+34b_0^2b_1^4 ----------------------------------------------------------------------- 8b_0^4b_1^2+50b_0^3b_1^3-9b_0^2b_1^4 ----------------------------------------------------------------------- 27a_0b_0^4b_1+23a_1b_0^4b_1-35a_0b_0^3b_1^2-19a_1b_0^3b_1^2+42a_0b_0^2b ----------------------------------------------------------------------- _1^3-23a_1b_0^2b_1^3-29a_0b_0b_1^4+14a_1b_0b_1^4 ----------------------------------------------------------------------- -27a_0b_0^4b_1+12a_1b_0^4b_1+23a_0b_0^3b_1^2-28a_1b_0^3b_1^2+34a_0b_0^ ----------------------------------------------------------------------- 2b_1^3+10a_1b_0^2b_1^3-36a_0b_0b_1^4+40a_1b_0b_1^4 ----------------------------------------------------------------------- -20a_0b_0^4b_1-2a_1b_0^4b_1+41a_0b_0^3b_1^2-46a_1b_0^3b_1^2-8a_0b_0^2b_ ----------------------------------------------------------------------- 1^3-17a_1b_0^2b_1^3-42a_0b_0b_1^4-3a_1b_0b_1^4 ----------------------------------------------------------------------- -34a_0^2b_0^3b_1+25a_0a_1b_0^3b_1+26a_1^2b_0^3b_1-33a_0^2b_0^2b_1^2-29a ----------------------------------------------------------------------- _0a_1b_0^2b_1^2+4a_1^2b_0^2b_1^2+42a_0^2b_0b_1^3-45a_0a_1b_0b_1^3-5a_1^ ----------------------------------------------------------------------- 2b_0b_1^3-7a_0^2b_1^4+5a_0a_1b_1^4 ----------------------------------------------------------------------- 30a_0^2b_0^3b_1+22a_0a_1b_0^3b_1-28a_1^2b_0^3b_1-28a_0^2b_0^2b_1^2+32a_ ----------------------------------------------------------------------- 0a_1b_0^2b_1^2-29a_1^2b_0^2b_1^2-9a_0^2b_0b_1^3+35a_0a_1b_0b_1^3+a_1^2b ----------------------------------------------------------------------- _0b_1^3-17a_0^2b_1^4+45a_0a_1b_1^4-12a_1^2b_1^4 ----------------------------------------------------------------------- -50a_0^2b_0^3b_1-12a_0a_1b_0^3b_1-39a_1^2b_0^3b_1+35a_0^2b_0^2b_1^2+27a ----------------------------------------------------------------------- _0a_1b_0^2b_1^2-2a_1^2b_0^2b_1^2+48a_0^2b_0b_1^3+33a_0a_1b_0b_1^3-36a_1 ----------------------------------------------------------------------- ^2b_0b_1^3+33a_0^2b_1^4-23a_0a_1b_1^4-15a_1^2b_1^4 ----------------------------------------------------------------------- -17a_0^2b_0^3b_1-5a_0a_1b_0^3b_1-13a_1^2b_0^3b_1-34a_0^2b_0^2b_1^2+26a_ ----------------------------------------------------------------------- 0a_1b_0^2b_1^2+39a_1^2b_0^2b_1^2+38a_0^2b_0b_1^3-18a_0a_1b_0b_1^3+20a_1 ----------------------------------------------------------------------- ^2b_0b_1^3+47a_0^2b_1^4+47a_0a_1b_1^4-22a_1^2b_1^4 ----------------------------------------------------------------------- 22a_0^3b_0^2b_1-5a_0^2a_1b_0^2b_1+25a_0a_1^2b_0^2b_1-12a_1^3b_0^2b_1+ ----------------------------------------------------------------------- 32a_0^3b_0b_1^2+9a_0^2a_1b_0b_1^2+10a_0a_1^2b_0b_1^2-31a_1^3b_0b_1^2- ----------------------------------------------------------------------- 37a_0^3b_1^3+27a_0^2a_1b_1^3-15a_0a_1^2b_1^3+45a_1^3b_1^3 ----------------------------------------------------------------------- 30a_0^3b_0^2b_1-17a_0^2a_1b_0^2b_1-34a_0a_1^2b_0^2b_1-50a_1^3b_0^2b_1- ----------------------------------------------------------------------- 10a_0^3b_0b_1^2-10a_0^2a_1b_0b_1^2-16a_0a_1^2b_0b_1^2-37a_1^3b_0b_1^2+a ----------------------------------------------------------------------- _0^3b_1^3+43a_0^2a_1b_1^3-a_0a_1^2b_1^3-11a_1^3b_1^3 ----------------------------------------------------------------------- -28a_0^3b_0^2b_1-13a_0^2a_1b_0^2b_1+26a_0a_1^2b_0^2b_1-39a_1^3b_0^2b_1+ ----------------------------------------------------------------------- 15a_0^3b_0b_1^2+21a_0^2a_1b_0b_1^2-8a_0a_1^2b_0b_1^2+26a_1^3b_0b_1^2- ----------------------------------------------------------------------- 33a_0^3b_1^3+33a_0^2a_1b_1^3+6a_0a_1^2b_1^3-5a_1^3b_1^3 ----------------------------------------------------------------------- 23a_0^4b_0b_1+12a_0^3a_1b_0b_1-2a_0^2a_1^2b_0b_1-2a_0a_1^3b_0b_1-44a_1^ ----------------------------------------------------------------------- 4b_0b_1-20a_0^4b_1^2-29a_0^3a_1b_1^2+16a_0^2a_1^2b_1^2+2a_0a_1^3b_1^2- ----------------------------------------------------------------------- 42a_1^4b_1^2 27a_0^4b_0b_1-27a_0^3a_1b_0b_1-20a_0^2a_1^2b_0b_1+2a_0a_1^ ----------------------------------------------------------------------- 3b_0b_1+27a_1^4b_0b_1+18a_0^4b_1^2+19a_0^3a_1b_1^2+45a_0^2a_1^2b_1^2+ ----------------------------------------------------------------------- 12a_0a_1^3b_1^2+7a_1^4b_1^2 ----------------------------------------------------------------------- 23a_0^2b_0^4+27a_0a_1b_0^4-44a_1^2b_0^4+9a_0^2b_0^3b_1+5a_0a_1b_0^3b_1- ----------------------------------------------------------------------- 30a_1^2b_0^3b_1+8a_0^2b_0^2b_1^2-21a_0a_1b_0^2b_1^2+32a_1^2b_0^2b_1^2+ ----------------------------------------------------------------------- 41a_0^2b_0b_1^3-14a_0a_1b_0b_1^3-35a_1^2b_0b_1^3+25a_0^2b_1^4-20a_0a_1b ----------------------------------------------------------------------- _1^4+45a_1^2b_1^4 -a_0^2b_0^4+47a_0a_1b_0^4+27a_1^2b_0^4-21a_0^2b_0^3b_ ----------------------------------------------------------------------- 1+4a_0a_1b_0^3b_1-42a_1^2b_0^3b_1-42a_0^2b_0^2b_1^2+31a_0a_1b_0^2b_1^2+ ----------------------------------------------------------------------- 22a_1^2b_0^2b_1^2-3a_0^2b_0b_1^3+45a_0a_1b_0b_1^3-6a_1^2b_0b_1^3+21a_0^ ----------------------------------------------------------------------- 2b_1^4-42a_0a_1b_1^4-33a_1^2b_1^4 ----------------------------------------------------------------------- 43a_0^2b_0^4-a_0a_1b_0^4+23a_1^2b_0^4+50a_0^2b_0^3b_1-32a_0a_1b_0^3b_1+ ----------------------------------------------------------------------- 49a_1^2b_0^3b_1+35a_0^2b_0^2b_1^2+50a_0a_1b_0^2b_1^2+6a_1^2b_0^2b_1^2- ----------------------------------------------------------------------- 42a_0^2b_0b_1^3+3a_0a_1b_0b_1^3+20a_1^2b_0b_1^3-41a_0^2b_1^4-12a_0a_1b_ ----------------------------------------------------------------------- 1^4-a_1^2b_1^4 9a_0^3b_0^3-39a_0^2a_1b_0^3-37a_0a_1^2b_0^3-50a_1^3b_0^3 ----------------------------------------------------------------------- +43a_0^3b_0^2b_1-9a_0^2a_1b_0^2b_1-8a_0a_1^2b_0^2b_1+47a_1^3b_0^2b_1+ ----------------------------------------------------------------------- 43a_0^3b_0b_1^2+4a_0^2a_1b_0b_1^2+35a_0a_1^2b_0b_1^2+26a_1^3b_0b_1^2+ ----------------------------------------------------------------------- 46a_0^3b_1^3-38a_0^2a_1b_1^3-21a_0a_1^2b_1^3-29a_1^3b_1^3 ----------------------------------------------------------------------- -8a_0^3b_0^3-50a_0^2a_1b_0^3-42a_0a_1^2b_0^3-24a_1^3b_0^3+24a_0^3b_0^2b ----------------------------------------------------------------------- _1-49a_0^2a_1b_0^2b_1-32a_0a_1^2b_0^2b_1-25a_1^3b_0^2b_1-4a_0^3b_0b_1^2 ----------------------------------------------------------------------- +35a_0^2a_1b_0b_1^2-44a_0a_1^2b_0b_1^2+13a_1^3b_0b_1^2-5a_0^3b_1^3-25a_ ----------------------------------------------------------------------- 0^2a_1b_1^3+44a_0a_1^2b_1^3+9a_1^3b_1^3 ----------------------------------------------------------------------- -33a_0^3b_0^3-37a_0^2a_1b_0^3-23a_0a_1^2b_0^3-42a_1^3b_0^3+9a_0^3b_0^2b ----------------------------------------------------------------------- _1+16a_0^2a_1b_0^2b_1-29a_0a_1^2b_0^2b_1-23a_1^3b_0^2b_1+30a_0^3b_0b_1^ ----------------------------------------------------------------------- 2+45a_0^2a_1b_0b_1^2+27a_1^3b_0b_1^2-6a_0^3b_1^3+5a_0^2a_1b_1^3-44a_0a_ ----------------------------------------------------------------------- 1^2b_1^3+20a_1^3b_1^3 -17a_0^3b_0^3+9a_0^2a_1b_0^3-33a_0a_1^2b_0^3-8a_1 ----------------------------------------------------------------------- ^3b_0^3-46a_0^3b_0^2b_1-16a_0^2a_1b_0^2b_1+32a_0a_1^2b_0^2b_1+22a_1^3b_ ----------------------------------------------------------------------- 0^2b_1+38a_0^3b_0b_1^2-17a_0^2a_1b_0b_1^2-33a_0a_1^2b_0b_1^2+7a_1^3b_0b ----------------------------------------------------------------------- _1^2+24a_0^3b_1^3-7a_0^2a_1b_1^3+28a_0a_1^2b_1^3-6a_1^3b_1^3 ----------------------------------------------------------------------- -8a_0^3a_1b_0^2-50a_0^2a_1^2b_0^2-42a_0a_1^3b_0^2-24a_1^4b_0^2+25a_0^4b ----------------------------------------------------------------------- _0b_1+3a_0^3a_1b_0b_1-43a_0^2a_1^2b_0b_1-15a_0a_1^3b_0b_1-46a_1^4b_0b_1 ----------------------------------------------------------------------- +18a_0^4b_1^2+7a_0^3a_1b_1^2-50a_0^2a_1^2b_1^2-22a_0a_1^3b_1^2+49a_1^4b ----------------------------------------------------------------------- _1^2 -33a_0^3a_1b_0^2-37a_0^2a_1^2b_0^2-23a_0a_1^3b_0^2-42a_1^4b_0^2+ ----------------------------------------------------------------------- 17a_0^4b_0b_1-35a_0^3a_1b_0b_1+45a_0^2a_1^2b_0b_1+50a_0a_1^3b_0b_1-48a_ ----------------------------------------------------------------------- 1^4b_0b_1+36a_0^4b_1^2-11a_0^3a_1b_1^2-3a_0^2a_1^2b_1^2+21a_0a_1^3b_1^2 ----------------------------------------------------------------------- -9a_1^4b_1^2 3a_0^4b_0^2+20a_0^3a_1b_0^2+24a_0^2a_1^2b_0^2+4a_0a_1^3b_0 ----------------------------------------------------------------------- ^2-41a_1^4b_0^2-16a_0^4b_0b_1+a_0^3a_1b_0b_1+22a_0^2a_1^2b_0b_1+17a_0a_ ----------------------------------------------------------------------- 1^3b_0b_1+3a_1^4b_0b_1-20a_0^4b_1^2-50a_0^3a_1b_1^2-46a_0^2a_1^2b_1^2- ----------------------------------------------------------------------- 27a_0a_1^3b_1^2+5a_1^4b_1^2 ----------------------------------------------------------------------- -17a_0^3a_1b_0^2+9a_0^2a_1^2b_0^2-33a_0a_1^3b_0^2-8a_1^4b_0^2-28a_0^4b_ ----------------------------------------------------------------------- 0b_1+7a_0^3a_1b_0b_1-32a_0^2a_1^2b_0b_1+39a_0a_1^3b_0b_1+36a_1^4b_0b_1+ ----------------------------------------------------------------------- 17a_0^4b_1^2-12a_0^3a_1b_1^2+4a_0^2a_1^2b_1^2+40a_0a_1^3b_1^2+16a_1^4b_ ----------------------------------------------------------------------- 1^2 9a_0^3a_1b_0^2-39a_0^2a_1^2b_0^2-37a_0a_1^3b_0^2-50a_1^4b_0^2+48a_0 ----------------------------------------------------------------------- ^4b_0b_1+20a_0^3a_1b_0b_1-a_0^2a_1^2b_0b_1-33a_0a_1^3b_0b_1-38a_1^4b_0b ----------------------------------------------------------------------- _1+22a_0^4b_1^2+50a_0^3a_1b_1^2-4a_0^2a_1^2b_1^2+16a_0a_1^3b_1^2-37a_1^ ----------------------------------------------------------------------- 4b_1^2 3a_0^4a_1b_0+20a_0^3a_1^2b_0+24a_0^2a_1^3b_0+4a_0a_1^4b_0-32a_0^ ----------------------------------------------------------------------- 4a_1b_1+14a_0^3a_1^2b_1+3a_0^2a_1^3b_1+43a_0a_1^4b_1 ----------------------------------------------------------------------- -17a_0^3a_1^2b_0+9a_0^2a_1^3b_0-33a_0a_1^4b_0+3a_0^4a_1b_1+32a_0^3a_1^ ----------------------------------------------------------------------- 2b_1-48a_0^2a_1^3b_1-11a_0a_1^4b_1 ----------------------------------------------------------------------- 9a_0^3a_1^2b_0-39a_0^2a_1^3b_0-37a_0a_1^4b_0+44a_0^4a_1b_1+16a_0^3a_1^ ----------------------------------------------------------------------- 2b_1-7a_0^2a_1^3b_1+4a_0a_1^4b_1 3a_0^4a_1^2+20a_0^3a_1^3+24a_0^2a_1^4 ----------------------------------------------------------------------- -17a_0^3a_1^3+9a_0^2a_1^4 9a_0^3a_1^3-39a_0^2a_1^4 | o11 : Matrix i12 : basis({0,0}, image E^1 .dd_{1,-2}) -- image 2-dimensional o12 = {-11, 0} | 0 0 | {-1, -10} | 0 0 | {-8, -1} | 0 0 | {-8, -1} | 0 0 | {-8, -1} | 0 0 | {-7, -2} | 0 0 | {-7, -2} | 0 0 | {-7, -2} | 0 0 | {-6, -3} | 0 0 | {-6, -3} | 0 0 | {-6, -3} | 0 0 | {-6, -3} | 0 0 | {-5, -4} | 0 0 | {-5, -4} | 0 0 | {-5, -4} | 0 0 | {-4, -5} | 0 0 | {-4, -5} | 0 0 | {-6, -3} | 0 0 | {-6, -3} | 0 0 | {-6, -3} | 0 0 | {-5, -4} | 0 0 | {-5, -4} | 0 0 | {-5, -4} | 0 0 | {-5, -4} | 0 0 | {-4, -5} | 0 0 | {-4, -5} | 0 0 | {-4, -5} | 0 0 | {-4, -5} | 0 0 | {-4, -5} | 0 0 | {-3, -6} | 0 0 | {-3, -6} | 0 0 | {-3, -6} | 0 0 | {-2, -7} | a_1^2b_0^4b_1^3 a_1^2b_0^3b_1^4 | {-2, -7} | 0 0 | {-2, -7} | 0 0 | o12 : Matrix i13 : basis({0,0}, ker E^1 .dd_{1,-2}) -- map is injective o13 = 0 o13 : Matrix i14 : basis({0,0}, target E^1 .dd_{1,-2}) -- target 2-dimensional o14 = {-8, -7} | a_0^4a_1^4b_0^4b_1^3 a_0^4a_1^4b_0^3b_1^4 | o14 : Matrix i15 : basis({0,0}, source E^1 .dd_{1,-2}) -- source 2 dimensional o15 = {-11, 0} | 0 0 | {-1, -10} | 0 0 | {-8, -1} | 0 0 | {-8, -1} | 0 0 | {-8, -1} | 0 0 | {-7, -2} | 0 0 | {-7, -2} | 0 0 | {-7, -2} | 0 0 | {-6, -3} | 0 0 | {-6, -3} | 0 0 | {-6, -3} | 0 0 | {-6, -3} | 0 0 | {-5, -4} | 0 0 | {-5, -4} | 0 0 | {-5, -4} | 0 0 | {-4, -5} | 0 0 | {-4, -5} | 0 0 | {-6, -3} | 0 0 | {-6, -3} | 0 0 | {-6, -3} | 0 0 | {-5, -4} | 0 0 | {-5, -4} | 0 0 | {-5, -4} | 0 0 | {-5, -4} | 0 0 | {-4, -5} | 0 0 | {-4, -5} | 0 0 | {-4, -5} | 0 0 | {-4, -5} | 0 0 | {-4, -5} | 0 0 | {-3, -6} | 0 0 | {-3, -6} | 0 0 | {-3, -6} | 0 0 | {-2, -7} | a_1^2b_0^4b_1^3 a_1^2b_0^3b_1^4 | {-2, -7} | 0 0 | {-2, -7} | 0 0 | o15 : Matrix

An alternative way to compute the connecting morphism is

 i16 : prune connectingMorphism(Hom(G, f), - 2) ; o16 : Matrix i17 : prune connectingMorphism(Hom(G, f), - 2) == E^1 .dd_{1, -2} o17 = true