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k is a perfect field. X and Y are two regular varieties over k. Does their fiber product over k remain to be regular?

Note: When k is algebraically closed it's true by Jacobian criterion. When k is not perfect there's counter-example.

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Just out of curiosity, what's the counterexample? –  S. Carnahan Oct 23 '09 at 6:02
    
Let the field be F_2(t). The two rings are A=F_2(t)[x,y]/(y^2+x^3+tx), B=F_2(t,s)/(s^2-t), which is actually a field. –  Taisong Jing Oct 24 '09 at 5:34
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up vote 5 down vote accepted

The answer is yes. Indeed, over a perfect field the notions of smooth and regular coincide so it follows from the fact that base change and composition preserve smoothness.

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