Many mathematicians know that Lewis Carroll was quite a good mathematician, who wrote about logic (paradoxes) and determinants. He found an expansion formula, which bears his real name (Charles Lutwidge) Dodgson. Needless to say, L. Carroll was his pseudonym, used in literature.

Another (alive) mathematician writes under his real name and under a pseudonym (John B. Goode). (That person, by the way, is Bruno Poizat: it's no secret, even MathSciNet knows it.)

What other mathematicians (say dead ones) had a pseudonym, either within their mathematical activity, or in a parallel career ?

Of course, don't count people who changed name at some moment of their life because of marriage, persecution, conversion, and so on.

**Edit**.
The answers and comments suggest that there are at least four categories of pseudonyms, which don't exhaust all situations.

- Professional mathematicians, who did something outside of mathematics under a pseudonym (F. Hausdorff -
*Paul Mongré*, E. Temple Bell -*John Taine*), - People doing mathematics under a pseudonym, and something else under their real name (Sophie Germain -
*M. Le Blanc*, W. S. Gosset -*Student*)), - Professional mathematicians writing mathematics under both their real name and a pseudonym (B. Poizat -
*John B. Goode*), - Collaborative pseudonyms (
*Bourbaki, Blanche Descartes*)