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seen Oct 3 at 13:23

Feb
11
revised shortcut for finding a inverse of matrix
added 3x3 explanation
Feb
11
comment shortcut for finding a inverse of matrix
Yes...but...it involves the determinant of the 3x3 and all the 2x2 submatrices. I thought that that isn't much of a trick or shortcut; it seems about the same complexity as just plodding through row/column operations to convert the 3x3 into an identity matrix and applying those operations to an identity matrix at the same time. Of course, if there's an expectation that the determinant is 1, then maybe it's appropriate. Also, be warned that the row/column operations are 'meaningful' (you see that they are computing the inverse) but the 'trick' is just blind application of a formula.
Feb
11
answered shortcut for finding a inverse of matrix
Feb
10
comment “Closed” form for $\sum \frac{1}{n^n}$
Right. It's just a clever saying, especially considering that the freshman's dream happens all the time.
Feb
10
comment Upper bound/exact length of decimal expansion of simple fraction
What's the length of the repeating part in the second case?
Feb
10
comment Calculating the formula for a graph
are you trying to justify a formula for that page that is given somewhere else? Or are you trying to come up with a formula out of the blue? Are you trying to fit a set of points on that page loosely (where you formula just has to get close to your points, using regression) or exactly (where the function goes exactly through each given point)?
Feb
10
answered What should be in every grad student's library?
Feb
9
answered For which values of $\alpha$ does $\int_0^1\frac{1-x^\alpha}{1-x}\mathrm dx$ converge?
Feb
9
answered What is the simplification of $\sin^2 x/(1+ \sin^2 x +\sin^4 x +\sin^6 x + \cdots)$?
Feb
9
answered The elementary coordinate geometry of polynomials? Of rational expressions? Of radicals?
Feb
9
comment Some expectation values for a Gamma distribution
wow. Really? Statistical manipulations are so inscrutable. How do you go about figuring out those answers?
Feb
7
comment The Hexagonal Property of Pascal's Triangle
@milcak: Check out math.stackexchange.com/questions/15505/combinatorial-identity for a similarly styled combinatorial proof.
Feb
7
comment The Hexagonal Property of Pascal's Triangle
@milcak: I added a paragraph stating more clearly the combinatorial part. I am not using the arithmetic of factorials in my proof, simply using the combinatorial interpretation of binomial coefficients as counting subsets and counting one situation in two different ways.
Feb
7
revised The Hexagonal Property of Pascal's Triangle
added paragraph repeating combinatorial explanation more clearly.
Feb
7
comment Proof of Statements involving Sets
@1337holiday: the steps in Henry's proof were (probably) not motivated by any direct foreknowledge that that particular step was the way to go, but more likely just using the strategy of simplifying (or translating out) the '$\rightarrow$' (which is a useful strategy because the identities involving '$\land$' and '$\lor$' are easier to manipulate.
Feb
7
comment Intuitionism and circles
Likewise, finitism is orthogonal to intuitionism (even though there are common directions in skepticism of mainstream mathematical methods).
Feb
7
answered The Hexagonal Property of Pascal's Triangle
Feb
7
comment Factorial and exponential dual identities
I just found this: for a function $f(n)$, its ordinary generating function $g(x)$ and its exponential generating function $G(x)$, there is the relation $g(x) = \int_0^{\infty} e^{-u} G(x u) du$. It seems like it should be obvious but I just don't see the manipulation that connects this relation with the dual identities (it's obvious how it applies to the second identity)
Feb
6
answered What is the current status of Vinay Deolalikar's proof that P is not equal to NP
Feb
6
answered Linear Programming Books