Esteban Crespi
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 Dec 9 revised Finding a matrix knowing determinant edited body Nov 28 revised On a generalization for $\sum_{d|n}rad(d)\phi(\frac{n}{d})$ and related questions added 1275 characters in body Nov 27 revised Solve the pde : $x^2(y-z)\,\partial_x(z)+y^2(z-x)\,\partial_y(z)=z^2(x-y)$ mathjax and typo Sep 24 revised Factorization in a Group Added a reference and modification of the answer. Mar 3 revised Is $53$ expressible in this form? added 551 characters in body Sep 26 revised Infinitely many primes congruent to 1 mod prime The proof was incomplete Sep 5 revised Number of pairs $(i, j)$ where $1\leq i < j \leq N$ such that $i|j$ added 148 characters in body Sep 5 revised Number of pairs $(i, j)$ where $1\leq i < j \leq N$ such that $i|j$ Improved the image and hopefully the explanation Apr 6 revised Comparing sums of surds without any aids added 53 characters in body Mar 31 revised Algorithm to find representation of an element of field extension $\mathbb{Q}(q)$ in the form $\sum a_i q^i$ added 125 characters in body Mar 16 revised Partial fraction decomposition of p'/p Removed the c Mar 14 revised Reducing simultaneously a pair of fractions $\frac{a^2}{b},\frac{ a^3}{c}$ using only gcds EDIT: Changed to make more clear the question. Mar 14 revised Reducing simultaneously a pair of fractions $\frac{a^2}{b},\frac{ a^3}{c}$ using only gcds Sorry the method I give is not correct Dec 22 revised Generalizing Ramanujan's proof of Bertrand's Postulate: Can Ramanujan's approach be used to show a prime between $4x$ and $5x$ for $x \ge 3$ minor edit Jun 21 revised How prove that:$\varphi(2)+\varphi(3)+\varphi(4)+\cdots+\varphi(n)\ge\frac{n(n-1)}{4}+1$ I have rewritten the proof as the original was very confusing and contained some incorrect statements. Jun 21 revised How prove that:$\varphi(2)+\varphi(3)+\varphi(4)+\cdots+\varphi(n)\ge\frac{n(n-1)}{4}+1$ deleted 2 characters in body Dec 1 revised Galois group and the Quaternion group added 1 characters in body Dec 1 revised Galois group and the Quaternion group Added explanation Nov 3 revised Half the rationals? Added a generalization Oct 15 revised Tiling a minimal perimeter region with $n$ unit squares Sketch of proof