This post is to be filed in the “useless but fun” category. A friend of mine was doing some Hadoopy stuff a few days ago, experimenting with rather large sparse matrices and their products. Long story short, we ended up wondering how sparse the product of 2 sparse matrices should be.

A sparse matrix is a matrix which is primarily filled with zeros. The product of two matrices involves computing the dot product of each row of the first one, with each column of the second one. There is clearly a relationship between how dense the two matrices are, and how dense the result should be. As an obvious illustration, if we consider 2 matrices populated only with zeroes – as sparse as it gets – their product is obviously 0. Conversely, two matrices populated only with ones – as dense as it gets – will result in a “completely dense” matrix. But… what happens in between?

Should I expect the product of 2 sparse matrices to be more, or less dense than the original matrices? And does it depend on how large the matrices are? What would be your guess?

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