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Dec
22
comment Kings on a chessboard
@Ross, only if they're at (4,5) and (5,4).
Dec
22
comment Kings on a chessboard
@Serkan, I'm not sure what you're counting there. The more obvious criticism is that the last sentence of my comment talks about "reducing" to a problem class which already contains the original problem.
Dec
22
answered Kings on a chessboard
Dec
20
comment Kings on a chessboard
Not a complete answer, but it might help: divide the board into 9 $2\times 2$ tiles. Each such tile can contain at most one king, so this gives an easy (but very loose) upper bound of $5^9$. Moreover, this approach reduces the problem to a 2D analogue of combinatorics on words: counting grids which avoid forbidden subgrids.
Dec
15
comment Calculate how many ways to get change of 78
What's wrong with the various programs that people supplied you in answer to your earlier question?
Dec
10
comment combinatorics - fixed point permutations
Wolfram Alpha suggests not. If there were a closed form then Petkovšek's algorithm should find it, and I would be very surprised if Alpha doesn't implement it.
Dec
10
reviewed Leave Open How many times do you write the digit 4 when writing all the numbers from 10 to 100?
Dec
9
reviewed Leave Open How to visualize $SO(4) \simeq SO(3)\bigotimes SO(3)$
Dec
9
reviewed Close Limit n->infity nth root (4^n + 7^n)
Dec
9
reviewed Leave Open Choice of the right isomorphisms
Dec
7
comment Polynomials with rational zeros
In addition to saying what you've tried - homework is given to help you learn by doing - it would help to say where you encountered this. What theorems did your course just cover?
Dec
4
comment Closed form expression for unusual sum of binomial coefficients
The way computer algebra systems derive the closed form expression is by using knowledge about what the answer looks like. Specifically, if your expression has an indefinite sum then it's the term multiplied by a rational polynomial, and it's possible to bound the degrees of the numerator and denominator. See Gosper's algorithm
Dec
2
reviewed Close Which of those are isomorphism
Dec
2
reviewed Close Conjecture about some rings and roots of unity.
Dec
2
comment Determining Ambiguity in Context Free Grammars
An even simpler ambiguous string is $()$.
Nov
30
reviewed Close Complete the square.
Nov
30
revised What is the minimum number of locks on the cabinet that would satisfy these conditions?
Add missing "fi" - I suspect this was copy-pasted from a PDF which used a ligature
Nov
28
reviewed No Action Needed Cumulative distribution function word problem
Nov
27
answered 9-Rook problem in 3D / weak version of sudoku
Nov
27
reviewed Approve suggested edit on Ice cream vendor problem