Trivial cycles

[crvs]

Hi, I was wondering if there is a simple way of checking if a cycle is trivial or not. I have been trying to use m.homologous(c,z) where c is a chain that contains the cycle I want to check (a simple closed curve) is trivial and z is simply d.Chain([]). For some reason the process seems to hang there indefinitely when I try to do this.

Another thing that I was wondering is what coefficient field is being used by default, because I need to know if I need to take orientation into account when creating a chain.

[mrzv]

Comparing to d.Chain([]) is the correct approach. I just checked, and it works here. Can you show me an example, where it's failing, so we can figure it out?

homology_persistence() uses Z mod 2 by default.

[crvs]

Maybe I'm doing something wrong, or maybe I just didn't give it enough time, I'll try to run it for longer, and if it still doesn't halt I will try to generate a toy example tomorrow.

[anvarava]

Hi! I am having a similar issue. Seems like the order of simplices in a chain matters -- is this correct? When I am trying to run the following code, I am getting a segmentation fault:

import dionysus as d
simplices = [([0], 5), ([1], 1), ([2], 3), ([0, 1], 5), ([0, 2], 6), ([1, 2], 3)]
f = d.Filtration()
for s, t in simplices: f.append(d.Simplex(s,t))
f.sort()
m = d.homology_persistence(f)
cycle = [ [0, 2], [1, 2],[0, 1]]
indices = [f.index(d.Simplex(list(s))) for s in cycle]
#indices.sort()
chain = d.Chain([(1,i) for i in indices])
z = d.Chain([])
print(m.homologous(chain, z))

If I sort the indices (ascending order), everything works well. Am I doing something wrong?

Thanks!

[mrzv]

Ah, that's a very good catch. The cycles you give to homologous() were indeed expected to be sorted. I just pushed a fix that sorts them within homologous(). If you install from this repo, you should get it. Can you test and see if it works for you? (Otherwise, the fix you give — sorting indices by hand — also works.)