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In Linear Algebra and Its Applications, David Lay writes, "the dimension of the null space is sometimes called the nullity of A, though we will not use the term." He then goes on to specify "The Rank Theorem" as "rank A + dim Nul A = n" instead of calling it the the rank-nullity theorem and just writing "rank A + nullity A = n".

Naturally, I wonder why he goes out of his way to avoid using the term "nullity." Maybe someone here can shed light....

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    http://en.wikipedia.org/wiki/Nullity2012-07-31
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    Perhaps if he used nullity, he would feel obliged to call the dimension of the column and row spaces the columnity and rowity, respectively.2012-07-31
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    I think it is sort of a silly term myself. The dimension of the kernel should have a more dignified name; it is a very important dimension.2012-07-31
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    Are you sure that the author doesn't use Nul $A$ as shorthand for the nullspace of $A$? If so, then the statement seems reasonable.2012-08-30
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    The word “nullity” is best applied to individual humans.2012-09-29
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    @Gerry: that would be a calamity. `:-)`.2012-10-31
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    I would have thought that nullity is perfectly ok. But at least four people whose opinions I regard highly apparently disagree. Making a mental note here. My excuse is that I have not encountered the word nullity in a context other than linear algebra, so the word is "just a word" for me. Apparently the word is not loaded with any undignified overtones for an ESL-user like me.2013-04-08

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