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For questions requesting examples of a certain structure or phenomenon

20 votes

Examples of algorithms requiring deep mathematics to prove correctness

How about the following problem: Given an integer $n$, how many ways are there to write it as the sum of $k$ squares? Or, equivalently, in $\mathbb{R}^k$, how many lattice points (in the standard int …
6 votes

Example s.t. the unbased loop-space is not $\Omega X \times X$

Often, as in these examples, you already see the difference in rational cohomology, which can usually be computed more easily using rational homotopy theory; a Sullivan minimal model of $X$ gives a minimal …
Lennart Meier's user avatar
40 votes

‘Naturally occurring’ $K(\pi, n)$ spaces, for $n \geq 2$

$\DeclareMathOperator\B{B}\newcommand\TOP{\mathrm{TOP}}\newcommand\PL{\mathrm{PL}}\newcommand\BTOP{{\B}\TOP}\newcommand\BPL{{\B}\PL}$Let $\BTOP$ and $\BPL$ be the classifying spaces of topological/PL- …
Lennart Meier's user avatar