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May
7
answered Let $a,b$ be relative integers such that $2a+3b$ is divisible by $11$. Prove that $a^2-5b^2$ is also divisible by $11$.
May
4
reviewed Close Famous Problems the Experts Could not Solve
May
4
reviewed Leave Open What is meant by a “$S$-strictly diagonally dominant matrix” in the book 'Geršgorin and His Circles'
May
4
reviewed Close A nice set of squares.
Apr
30
reviewed Close What are numbers?
Apr
30
reviewed Close Proof verification for proving $\forall n \ge 2, 1 + \frac1{2^2} + \frac1{3^2} + \cdots + \frac1{n^2} < 2 − \frac1n$ by induction
Apr
30
reviewed Leave Open Help with finding range and equation of a position vector (projectile)
Apr
29
reviewed Leave Open how to combine angle rotations along different axes into one rotation along a single vector
Apr
29
reviewed Leave Open Statistics Problems, I don't understand what this means..
Apr
29
answered Collection all cards in a cardgame
Apr
29
comment If $n$ is composite, then $((n-1)!)^2 \equiv 0 \pmod n$
4 is the only counterexample: see e. g. Benoit Cloitre's comment at oeis.org/A046022
Apr
29
reviewed Looks OK Evaluate $\lim _{n\to \infty }\int_1^2\:\frac{x^n}{x^n+1}dx$
Apr
29
reviewed Looks OK Show that there exists an idempotent element such that $Ra=Re$ holds for ring $R$
Apr
29
reviewed Approve $\int_{-1}^1 \int_{-1}^1 \sqrt{\frac{1+x-y-xy}{1-x+y-xy}} \, dx\,dy $
Apr
29
reviewed Approve Vector Space Basis' Proof
Apr
29
reviewed Leave Open High-School level probability and logic problem
Apr
29
reviewed Close How to construct a product set whose complement is not a product set?
Apr
29
reviewed Close Let A be a countable set of points on the plane. Prove that the remaining part of the plane is connected.
Apr
29
reviewed Leave Open Mean and variance of a random variable whose distribution is Pr {X = k} = Binomial [N-k, n] *Binomial [k-1,n] /Binomial [N, 2n+1] for k=n+1, …, N-n
Apr
29
reviewed Leave Open How to understand why $x^0 = 1$, where $x$ is any real number?