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Apr
24
comment Run of $N$ successes before run of $k$ failures
Google found a math.SE question that was 45mins old when you searched? Wow.
Apr
21
comment How to rotate n individuals at a dinner party so that every guest meets every other guests
You might want to check out en.wikipedia.org/wiki/Combinatorial_design
Apr
21
awarded  Quorum
Apr
20
comment Calculate which day of the week a date falls in using modular arithmetic
I actually teach this algorithm in my algorithms class; it's useful in real life to be able to do this!
Apr
16
awarded  Good Question
Apr
16
comment Deleting any digit yields a prime… is there a name for this?
@Martijn I intended that the original number be prime as well; edited to clarify this.
Apr
16
revised Deleting any digit yields a prime… is there a name for this?
clarified that original number must also be prime
Apr
15
answered Deleting any digit yields a prime… is there a name for this?
Apr
15
comment Deleting any digit yields a prime… is there a name for this?
By the way, these number don't look random at all: they are heavily biased toward integers with repeated consecutive digits. Take 711110111, for example: such numbers appear often in our list because if deleting one 1 in a group yields a prime, then obviously deleting any other 1 in that group does as well.
Apr
15
comment Deleting any digit yields a prime… is there a name for this?
Here is heuristic evidence that there are infinitely many of these numbers: let $c(n)$ be the number of integers in $P$ with $n$ decimal digits. A quick computer program shows $c(2)=4$, $c(3)=11$, $c(4)=14$, $c(5)=16$, $c(6)=18$, $c(7)=13$, $c(8)=14$, $c(9)=18$; therefore $P$ is infinite. I'm joking, of course, but I would find it remarkable if such a list ended, in spite of your nice argument above.
Apr
15
comment Deleting any digit yields a prime… is there a name for this?
I see no reason to exclude 0 as a digit... seems rather artificial: there are infinitely-many primes with a 0 digit in their decimal expansion (101 is the first, then cf Dirichlet).
Apr
15
awarded  Nice Question
Apr
15
asked Deleting any digit yields a prime… is there a name for this?
Apr
12
comment Simple probability question, balls and bins
Douglas: Thanks for adding your answer to the mix. But I don't follow: you have your summation indexed by "$k$ bins known to be empty". How do you evaluate a summation based on what "is known"? I would suggest you improve your answer by clarifying this. Also, your answer doesn't have an answer explicitly stated anywhere (that I can find). I think it would be an improvement to highlight the answer for "exactly one bin empty" rather than leaving it to the reader to "multiply by $m$ and divide by $m^n$" and simplify. (Unless you were intending to give only a hint?!) Finally...
Apr
11
answered Decryption Problem
Apr
11
accepted How many sides does a circle have?
Apr
10
comment Difficult integral?
What have you tried? Where did you get stuck? Where is this problem from? Did you notice that $1-x^4 = (1-x^2)(1+x^2)$?
Apr
10
comment What does it mean to say a language is context-free?
See en.wikipedia.org/wiki/Context-free_language
Apr
9
comment How many sides does a circle have?
@Douglas Zare: C?
Apr
9
awarded  Nice Question