

A077762


Number of ways of pairing the squares of the numbers 1 to n with the squares of the numbers n+1 to 2n such that each pair sums to a prime. Because an odd square must always be added to an even square to obtain a prime, this sequence is the product of A077763 and A077764.


2



1, 1, 0, 1, 2, 0, 1, 1, 4, 8, 0, 8, 42, 28, 140, 616, 836, 180, 1416, 2542, 10960, 96048, 242204, 367587, 923949, 1145430, 2622420, 19081728
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OFFSET

1,5


COMMENTS

Apparently, for n>11, there seems always to be a pairing possible. Note that all primes have the 4k+1 form. By the 4k+1 theorem, such a prime has a unique representation as the sum of two squares.


LINKS

Table of n, a(n) for n=1..28.
L. E. Greenfield and S. J. Greenfield, Some Problems of Combinatorial Number Theory Related to Bertrand's Postulate, J. Integer Sequences, 1998, #98.1.2.


FORMULA

a(n) = permanent(m), where the n X n matrix m is defined by m(i,j) = 1 or 0, depending on whether i^2 + (j+n)^2 is prime or composite, respectively.  T. D. Noe, Feb 10 2007


EXAMPLE

a(5) = 2 because there are two ways: (1,4,9,16,25) + (36,49,100,81,64) = (37,53,109,97,89) and (1,4,9,16,25) + (100,49,64,81,36) = (101,53,73,97,61).


MATHEMATICA

lst1*lst2 (* which are defined in A077763 and A077764 *)


CROSSREFS

Cf. A000348, A070897, A077763, A077764.
Sequence in context: A292712 A247504 A235955 * A244677 A243986 A085496
Adjacent sequences: A077759 A077760 A077761 * A077763 A077764 A077765


KEYWORD

hard,nonn


AUTHOR

T. D. Noe, Nov 15 2002


STATUS

approved



