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 Mar 2 comment Gaps between primes Sure it could work. But the statement we're trying to prove is that there are infinitely many twin primes, not that there could be infinitely many. Jul 1 comment Why are the only division algebras over the real numbers the real numbers, the complex numbers, and the quaternions? There is a very nice and simple proof given by Jyrki Lahtonen here. Jul 1 revised Proving that $\mathbb R^3$ cannot be made into a real division algebra (and that extending complex multiplication would not work) Made the title reflect the topic. Jul 1 suggested approved edit on Proving that $\mathbb R^3$ cannot be made into a real division algebra (and that extending complex multiplication would not work) Dec 20 awarded Constituent Dec 16 awarded Caucus Oct 27 comment open interval written as countable union of closed intervals Formal verification relies on the Archimedean property of $\mathbb{R}$. Oct 14 comment Notation: subscript vs. superscript for coordinate vector fields Hi, IMO the video is saying there are two ways to extract meaningful coordinates in an oblique coordinate system: taking orthogonal projection to oblique axes gives a covariant vector; parallel projections gives a contravariant vector. Neither is wrong or right by itself. The key is that inner product of covariant and contravariant gives an invariant independent of obliqueness of coordinate system. This invariant is exactly norm-squared when in orthogonal coordinates where covariant = contravariant. So both projections are part of understanding length. Does that help? @EricAuld Oct 14 comment Gauss Disq. Arithm. Translation Errata? +1 A bold project -- well done! Oct 1 awarded Yearling Sep 30 awarded Explainer Sep 24 awarded Autobiographer Sep 22 revised Notation: subscript vs. superscript for coordinate vector fields Added reference Sep 22 suggested rejected edit on Matrix rows notation Aug 12 revised Notation: subscript vs. superscript for coordinate vector fields Put picture before explanation. Aug 12 answered Notation: subscript vs. superscript for coordinate vector fields Aug 11 revised Notation: subscript vs. superscript for coordinate vector fields Revised the phrasing to avoid ambiguity by putting the descriptor before each example. Aug 11 suggested approved edit on Notation: subscript vs. superscript for coordinate vector fields Aug 11 comment Parenthesis vs brackets for matrices No, ll ll is a notation usually reserved for the norm of a matrix. Jul 7 revised Keep getting generating function wrong (making change for a dollar) Added the actual problem into title so it surfaces when searching for making change.