Condensed matrix function to find pairs

For a set of observations:

[a1,a2,a3,a4,a5]

their pairwise distances

d=[[0,a12,a13,a14,a15]
   [a21,0,a23,a24,a25]
   [a31,a32,0,a34,a35]
   [a41,a42,a43,0,a45]
   [a51,a52,a53,a54,0]]

Are given in a condensed matrix form (upper triangular of the above, calculated from scipy.spatial.distance.pdist ):

c=[a12,a13,a14,a15,a23,a24,a25,a34,a35,a45]

The question is, given that I have the index in the condensed matrix is there a function (in python preferably) f to quickly give which two observations were used to calculate them?

f(c,0)=(1,2)
f(c,5)=(2,4)
f(c,9)=(4,5)
...

I have tried some solutions but none worth mentioning :(