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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
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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