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Numerical algorithms for problems in analysis and algebra, scientific computation

5 votes

Well-balanced covering of transpositions in $n$ elements

$R_n \geq n/16$ can be obtained by starting from an arbitrary $f$ and then switching pairs of transpositions to get rid of any overlapping pairs whose images are too close to each other. Suppose $r(f …
Noam D. Elkies's user avatar
27 votes
Accepted

show that $ \frac{\Gamma(\frac{1}{24})\Gamma(\frac{11}{24})}{\Gamma(\frac{5}{24})\Gamma(\fra...

This formula can actually be proved using only properties of the Gamma function already known to Gauss, with no need to invoke special values of Dirichlet series. The relevant identities are $$ \Gamm …
Noam D. Elkies's user avatar
9 votes
Accepted

Alternating binomial Dirichlet series

Does this reduce to values of a known special function for arbitrary real (or complex) $s$? Answered by Johannes Trost in a comment: it's also known as a "Roman harmonic number". But this …
Noam D. Elkies's user avatar
5 votes
Accepted

Approximating high-dimensional integrals by low-dimensional ones

Here's how to average $e^{sx}$ over $A_t$. Let $r = t/2$, so at the $N$-th step of the construction of $A_t$ we have the disjoint union $A_t^{(N)}$ of $2^N$ intervals of length $r^N$ whose left endpo …
Noam D. Elkies's user avatar
11 votes
Accepted

Sparse approximation of the inverse of a sparse matrix

Not in general. An explicit and elementary counterexample is the sparse triangular matrix with $1$'s on the diagonal and $-1$'s just above it: the inverse is the triangular matrix with every entry …
Noam D. Elkies's user avatar