Hanul Jeon
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 Apr 25 comment How to prove $\mathop {\lim }\limits_{k \to \infty } \frac{1}{k}\sum\limits_{m = 1}^k {\frac{1}{m}} = 0$ What is your attempt? Apr 25 comment set theory simplificaiton (laws of set theory) $(Y-X)\cap X$ is the empty set. Apr 25 comment Is a set5 is countable if we can found a bijection between the set and a subset of natural number? In set theory the term "countable" has two different meaning. It may denote a set that has a bijection between the set and $\Bbb{N}$. However some people use the term "countable" to denote a set which is finite or equipotent to $\Bbb{N}$. Apr 25 revised $|G| = 12p$ with $p>5$ is not simple deleted 1 character in body; edited body; added 20 characters in body Apr 25 revised $|G| = 12p$ with $p>5$ is not simple Fix my proof.; added 11 characters in body; added 1 character in body; deleted 4 characters in body; edited body Apr 24 answered $|G| = 12p$ with $p>5$ is not simple Apr 24 revised how to give a truth value for the following formula Remove an irrelevant tag. Apr 24 revised Evaluate $\lim_{n\to \infty}{\sqrt{(1-\cos(1/n))\sqrt{(1-\cos(1/n))\dots}}}$ added 4 characters in body; edited title Apr 24 comment Evaluating $\lim\limits_{x \to -\infty} \sqrt{x^2 + 3x} - \sqrt{x^2 + x}$. Is Wolfram wrong or is it me? Possibly you make an error when calculating a square root. Most common mistake is taking $\sqrt{x^2} = x$ even if $x$ is negative. Mar 28 revised Probability of having $k$ similar elements in two subsets. added 17 characters in body Mar 28 comment real analysis proof related to functions Have you tried the contrapositive? Mar 28 revised real analysis proof related to functions edited tags Mar 25 revised Limits of finite structures - first order logic added 1 character in body Mar 22 comment Is it possible to formalize all mathematics in terms of ordinals only? You may interested in the class of all hereditarily ordinal definable sets, which forms an inner model of the ZFC. Mar 21 comment Is it possible to formalize all mathematics in terms of ordinals only? I guess the answer is negative, though we need a concrete formulation of the question. The reasons are: first, power set of some set (such as $\mathcal{P}(\Bbb{N})$.) does not have natural well-order. It is consistent with ZF that the power set of $\Bbb{R}$ is not well-orderable. Second, some set may not definable from ordinals (although we can assume that every set is ordinal-definable). Mar 20 comment isomorphic finitely generated group @hattabhawete I have no idea how to check this. Mar 20 revised isomorphic finitely generated group deleted 3 characters in body Mar 20 revised An elegant way to solve $\frac {\sqrt3 - 1}{\sin x} + \frac {\sqrt3 + 1}{\cos x} = 4\sqrt2$ added 24 characters in body Mar 20 answered isomorphic finitely generated group Mar 20 revised find $\lim_{n\to \infty}\frac{(-10)^n}{4^{2n+1}(n+1)}$ added 7 characters in body; edited title