How to cite sage in bibliography ?

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yes; see also this question: http://ask.sagemath.org/question/409/citing-sage

Before my post I have searched for "cite Sage" and even that "How do I cite Sage? " is part of citing-sage question no match happened. I tried to avoid asking bt I didn't find nothing with "cite sage" event that it exists.

if you try searching for just "cite", then I think you'll find the other question. I think including "sage" introduces too much noise in the search :) In any case, this is useful enough to be worth posting a few times.