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Comment by jdwg | Hacker News Reader

Cycles on (X, A) really can connect parts of A to other parts that look remote if you are forced to stay within A. This is visible in the long exact sequence; when the third map is nonzero, you have cycles in A that become boundaries when you allow chains in X.

Sorry, I do not see what you mean by "dual space," and I do not myself view chains as a analogous to functions.

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Thanks for drawing my attention to the long exact sequence*; if I'm interpreting it correctly, the injection at 𝛼 and projection at 𝛽 split, so we have a direct sum: H(X,∅) ≃ H(X,A) ⊕ H(A,∅)?

(never mind the function analogy, I was trying to handwave a quotient in the other direction but that would fall immediately out of the direct sum if I understand correctly: A ≃ (A⊕B)/B and B ≃ (A⊕B)/A?)

* which forms a Möbius band in its own way, because at H(A,∅) we feed it into 𝛼 as ∅ ⊕ Cycles(A) but get it out of 𝛾 as Cycles(A) ⊕ ∅, leading to a "twist"?

[Edit: are you aware of Dan Piponi's blogging?

http://blog.sigfpe.com/2006/08/algebraic-topology-in-haskell...

http://blog.sigfpe.com/2006/08/what-can-we-measure-part-i.ht...

http://blog.sigfpe.com/2010/01/target-enumeration-with-euler...

etc.]

The direct sum decomposition you mention doesn't always happen, as you can see in some of the examples in the calculator. It is closer to happening when \gamma is zero, and if \gamma is zero and we switch from integer coefficient to field coefficients, then it always happens!

obviously I need to play with the calculator more (and probably work the exercises) ... hope you don't mind if I get back to you with more Q's once I have them.

well, I love the subject, so I'm glad to discuss whatever! And let me know if all of the exercises are too hard. There should always be a couple of easy ones, but I don't know this audience very well.

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