MATH.STACKEXCHANGE

https://math.stackexchange.com/questions/247425/is-there-a-vector-space-that-cannot-be-an-inner-product-space

https://math.stackexchange.com/questions/2621446/is-the-fourier-series-always-the-best-approximation

https://math.stackexchange.com/questions/641492/conservative-fields-and-not-simply-connected-domains

https://math.stackexchange.com/questions/2635850/does-zero-curl-imply-a-conservative-field