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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? Apr 9 comment How many sides does a circle have? @Douglas Zare: C? Apr 9 awarded Nice Question