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I am having trouble to know what is the formal mathematical way to in which we define what is a sparse matrix. I know that a sparse matrix, is a matrix in which most of the elements are zero. But by most are we saying 50% of the elements are zero?

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    $\begingroup$ That's not really how people use sparsity. It is really an asymptotic notion: we are considering $n \times n$ matrices with, say, $o(n^2)$ nonzero entries as $n \to \infty$. $\endgroup$ – Qiaochu Yuan Dec 5 '17 at 1:13
  • $\begingroup$ I know that. I just thought that by now someone had come with a formal definition of sparsity of sparse matrix. $\endgroup$ – Pedro Martins Dec 5 '17 at 1:18
  • $\begingroup$ @QiaochuYuan: Your comment makes me curious -- as far as I know, sparse matrices arising from scientific computing problems (usually finite difference and finite element methods) have only $O(n)$ nonzero entries. Do you know of situations where sparse matrices with a superlinear number of nonzeros, say $O(n\log n)$, are of interest? $\endgroup$ – user856 Dec 5 '17 at 3:43
  • $\begingroup$ No, I just wanted to say the largest thing that made sense. $O(n)$ also made sense but I didn't want to preclude the possibility of, as you say, $O(n \log n)$ or something like that also making sense. $\endgroup$ – Qiaochu Yuan Dec 5 '17 at 4:00
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I don't think there is an 'official' or 'formal' break point where a graph becomes sparse. You generally speak of a sparse matrix when the sparsity of the matrix is noteworthy, and especially if it allows you to apply special computational techniques that only work on matrices with many zero entries.

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    $\begingroup$ Technically, computational techniques designed for sparse matrices will still work on dense matrices, they will just work very very slowly. $\endgroup$ – user856 Dec 5 '17 at 2:18

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