Nine lemma

 ( Homological Algebra ) 

In mathematics, the nine lemma (or 3×3 lemma) is a statement about commutative diagrams and valid in the category of groups and any abelian category.

Consider the commutative diagram to the right. We have 6 statements:

• If all columns as well as the two bottom rows are exact, then the top row is exact.
• If all columns as well as the two top rows are exact, then the bottom row is exact.
• If all columns as well as the top and bottom rows are exact, and $A\_2\; \backslash to\; C\_2$ is the zero morphism, then the middle row is exact.
• By symmetry, exchanging the words "row" and "column" gives 3 more true statements.

The nine lemma can be proved

by direct diagram chasing, or by applying the snake lemma (to the two bottom rows in the first case, and to the two top rows in the second case).

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Variants The sharp nine lemma

(1967). 9783540586623, Springer. ISBN 9783540586623

is slightly stronger.

Define a sequence $0\; \backslash to\; A\; \backslash to\; B\; \backslash to\; C\; \backslash to\; 0$ "left exact" iff $0\; \backslash to\; A\; \backslash to\; B\; \backslash to\; C$ is exact. Then:

• If all columns as well as the two bottom rows are left exact, then the top row is left exact.
• If all columns as well as the two bottom rows are left exact, and the first column and the middle row are short exact, then the top row is exact.

In Mathematics Made Difficult, Linderholm offers a satirical view of the nine lemma:

Draw a tic-tac-toe board... Do not fill it in with noughts and crosses... Instead, use curved arrows... Wave your hands about in complicated patterns over this board. Make some noughts, but not in the squares; put them at both ends of the horizontal and vertical lines. Make faces. You have now proved:

(a) the Nine Lemma
(b) the Sixteen Lemma
(c) the Twenty-five Lemma...

in which only (a) is a widely recognized theorem in homological algebra.

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