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2d
revised Splitting equilateral triangle into 5 equal parts
url fixed
2d
revised Splitting equilateral triangle into 5 equal parts
edited title
Apr
8
comment Proof of the binomial identity $\displaystyle\binom{m}{n}=\sum_{k=0}^{\lfloor n/2 \rfloor} 2^{1-\delta_{k,n-k}} \binom{m/2}{k} \binom{m/2}{n-k}$
See also Combinatorial interpretation for the identity $\sum\limits_i\binom{m}{i}\binom{n}{j-i}=\binom{m+n}{j}$? etc
Apr
8
comment Proof of the binomial identity $\displaystyle\binom{m}{n}=\sum_{k=0}^{\lfloor n/2 \rfloor} 2^{1-\delta_{k,n-k}} \binom{m/2}{k} \binom{m/2}{n-k}$
that's just a special case of en.wikipedia.org/wiki/Vandermonde%27s_identity
Feb
5
awarded  Notable Question
Jan
15
reviewed Close Solve the equation $2φ(x)=x $ for $x\in\mathbb N^+.$
Jan
14
awarded  Nice Question
Dec
30
reviewed Close The ring of real convergent sequences is Noetherian
Dec
29
reviewed Leave Closed Prove $(1+2+…+k)^2 = 1^3 + … + k^3$ using induction
Dec
25
comment Combinatorial explanation for why $n^2 = {n \choose 2} + {n+1 \choose 2}$
coloured squares also can be counted visually: mathoverflow.net/questions/8846/proofs-without-words/8847#8847
Dec
17
reviewed Close Why does $f:[a,b] \rightarrow \mathbb{R}$ need $f$ to be continuous on $[a,b]$ but only differentiable on $(a,b)$, in some theorems.
Dec
16
reviewed Leave Open What is the limit of $\frac{1}{\sqrt{n}}\sum_{k=1}^n\frac{1}{\sqrt{2k-1}+\sqrt{2k+1}}$?
Dec
16
reviewed Leave Open Vanishing Gaussian curvature
Dec
16
reviewed Looks OK Is an undefined equality vacuously true?
Dec
16
reviewed Looks OK What is the value of the finite sum?
Dec
16
reviewed Looks OK Finding the logarithm of a matrix?
Dec
14
comment How does one read $\bar{A}$ aloud in Russian?
«A с чертой» is frequently used (have never heard «A с надчеркиванием» in my life)
Dec
14
reviewed Leave Closed How does one read $\bar{A}$ aloud in Russian?
Dec
14
reviewed Close How do you pronounce (partial) derivatives?
Dec
14
reviewed Close Why is $\sinh$ often pronounced “shine”?