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Complexes :: isExact

isExact -- whether a complex is exact

Synopsis

Description

The complex $C$ is exact if and only if the homology group $H^i(C)$ is the zero module, for all $i$. If bounds are given, then true is returned if $H^i(C) = 0$ for all $lo \le i \le hi$.

A resolution $C$ is an exact complex except in homological degree 0. The augmented complex $C'$ is exact everywhere.

i1 : S = ZZ/101[a..d];
i2 : I = monomialCurveIdeal(S, {1,3,4})

                        3      2     2    2    3    2
o2 = ideal (b*c - a*d, c  - b*d , a*c  - b d, b  - a c)

o2 : Ideal of S
i3 : C = freeResolution I

      1      4      4      1
o3 = S  <-- S  <-- S  <-- S
                           
     0      1      2      3

o3 : Complex
i4 : prune HH C

o4 = cokernel | bc-ad c3-bd2 ac2-b2d b3-a2c |
      
     0

o4 : Complex
i5 : assert not isExact C
i6 : assert isExact(C, 1, infinity)
i7 : C' = cone inducedMap(complex(S^1/I), C)[1]

                                                   1      4      4      1
o7 = cokernel | bc-ad c3-bd2 ac2-b2d b3-a2c | <-- S  <-- S  <-- S  <-- S
                                                                        
     -1                                           0      1      2      3

o7 : Complex
i8 : prune HH C'

o8 = 0
      
     0

o8 : Complex
i9 : assert isExact C'

See also

Ways to use isExact :

For the programmer

The object isExact is a method function.