How to write an involution in Weyl group as a product of $2$-cycles?

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Something along these lines should do the job:

R = W.domain().roots()
perm = Word(map(t2.action,R)).standard_permutation() / Word(R).standard_permutation()
perm.cycle_tuples()

@max, thank you very much! But the result I got is [(1, 12), (2, 11), (3, 10), (4, 9), (5, 8), (6, 7)] not (1,4)(2,3). Do you know what is the problem? Thank you very much.

It depends on how you interpret t2 as a permutation. In the code above I consider it as a permutation of the 12 roots. If you have something else in mind, modify the code accordingly.

E.g. try to change .roots() to .simple_roots().

@max, thank you very much. I tried simple_roots() but it has errors. The t2 is the product of simple reflections $s_1s_2s_3s_1s_2s_1$.