OFFSET
1,4
COMMENTS
Let p be the n-th prime number. Put 1 to p^2 into a square array in order. Choose a set of primes such that there is one and only one in each row and column. This is equivalent to non-attacking rooks on prime-numbered squares. Then a(n) is the number of solutions for the n-th prime.
REFERENCES
Martin Erickson, Beautiful Mathematics, Mathematical Association of America, 2011, p. 6. The problem is called Transversal of primes.
LINKS
J. K. Andersen, W. Edwin Clark, Jud McCranie, Carlos Rivera, Emmanuel Vantieghem, Puzzle 649 (www.primepuzzles.net)
EXAMPLE
For n=2, p=3, and the only solution is {3,5,7}, so a(2) = 1.
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Jud McCranie, Aug 18 2012
STATUS
approved