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The dimension of a ring is defined as the length of a longest prime chain as usual.

Let $A,B$ be affine rings over a field $k$. Then $$\dim A\otimes_k B = \dim A + \dim B.$$ How can we prove or disprove this?

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Thank you. We can use Noether Normalization. But how? – Tom Sep 6 '12 at 0:15
up vote 5 down vote accepted

I think the easiest way to prove this uses Noether normalization. [See this handout of Hochster’s.] To give more of a hint, you know that there is a polynomial subalgebra $k[T_1, \dots, T_n] \subseteq A$ over which $A$ is finite, where $n = \dim A$. Similarly, say $B$ is finite over $k[S_1, \dots, S_m]$. Can you write down an $(n + m)$-variable polynomial subalgebra of $A \otimes_k B$? Is $A \otimes_k B$ finite over this subring? Why is that enough?

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Thank you very much! I understood it! – Tom Sep 6 '12 at 0:35
Nice answer, Dylan. – Georges Elencwajg Sep 6 '12 at 8:33

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