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### Good examples of "two is easy, three is hard" in computational sciences

MaxEnt distribution subject to equality constraints on cumulants is easy to compute for constraints on first 2 cumulants (closed form solution), hard for constraints on first 3 cumulants. In the ...
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The non-negative rank of an entrywise non-negative matrix $A\in\mathbb{R}^{m\times n}_{\geq 0}$, i.e., the minimum $r$ for which a factorization $A = BC$ exists with $B\in\mathbb{R}^{m\times r}_{\geq ... • 8,610 2 votes ### Good examples of "two is easy, three is hard" in computational sciences Given an integer N, it's easy enough to find m,n so that mn=N and ... 0 votes ### Good examples of "two is easy, three is hard" in computational sciences Predicting the behaviour of a pendulum is comparatively easy. For the simplification of a mathematical pendulum we know the analytic solutions and the numerical simulation of a single pendulum is ... • 1,976 3 votes ### What FEM solver should be used for matrix-valued FE spaces? This is a fairly standard elliptic problem (the operator is of the form$-\Delta - \text{grad}\,\text{div}\$). It is vector-valued (in fact matrix-valued) in the same way as the elasticity equations ...
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