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7h
comment How to show $\frac{19}{7}<e$
@Bernard No. $\sum_{n=0}^3\frac1{n!}=\frac83<\sum_{n=0}^4\frac1{n!}=\frac{65}{24}<\frac{19}7$
7h
revised Why is $R((X))$ defined as follows?
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7h
comment Why is $R((X))$ defined as follows?
Hm, I assume the downvote is because I did not restate the formal definitions of all those constructs?
7h
answered Why is $R((X))$ defined as follows?
7h
revised Is the series $\sum_{n=1}^\infty\frac{n^n}{n!e^n}$ divergent?
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7h
revised Is the series $\sum_{n=1}^\infty\frac{n^n}{n!e^n}$ divergent?
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8h
answered Is the series $\sum_{n=1}^\infty\frac{n^n}{n!e^n}$ divergent?
12h
comment Are there infinite self-locating strings in the decimal expansion of $\pi$?
The problem for $\frac19$ is simpler :)
12h
answered Show that $\sin(x) > \ln(x+1)$ for any $x \in (0,1)$
13h
comment How to show $\frac{19}{7}<e$
The Taylor method needs only $\sum_{n=0}^5\frac1{n!}=\frac{163}{60}$, which has a moderate denominator. $163\cdot 7-19\cdot 60 = 1$
13h
comment Prove that if $ab \equiv cd \pmod{n}$ and $b \equiv d \pmod n$ and $\gcd(b, n) = 1$ then $a \equiv c \pmod n$.
The gcd condition implies that $b$ (hence also $d$) is invertible
13h
comment ZFC and cardinality
What is even missing from the result, given those hints?
16h
comment Subgroups of $\mathbb{Q}/ \mathbb{Z}$
You mean all proper subgroups?
1d
comment Is $\Bbb Q[x]/(x^2+x)$ isomorphic to $\Bbb Q[x]/(x^2-x)$?
@mathshungry Because $x\mapsto -x$ takes $x^2+x$ to $x^2-x$. And yes for your interpretation.
1d
comment What should I have learnt as an undergraduate?
I'd say, You should know where to find information you may lack ...
1d
comment Prove that the sum of the squares of two odd integers cannot be the square of an integer.
This is quite fine. For completeness you might want to add that an even square must be the square of an even number / divisible by 4.-- Anyone more experienced might have remembered that odd squares are $\equiv 1\pmod 8$, hence the sum of two such is $\equiv 2\pmod 8$, which cannot be square. Your argument boils down to working $\pmod 4$, which is in fact sufficent
1d
answered Is $\Bbb Q[x]/(x^2+x)$ isomorphic to $\Bbb Q[x]/(x^2-x)$?
1d
revised Why formulate continuity in terms of pre-images instead of image?
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1d
awarded  Nice Answer
1d
comment is this $f(x)$ continuous at almost every point of an interval $[a, b]$.
Of course, why not?