A051239 Number of ways to arrange integers 1 through n so that the sum of each adjacent pair is prime, not counting reversals.

1, 1, 1, 4, 2, 8, 12, 30, 70, 664, 1072, 8768, 11648, 37108, 95772, 1059816, 2047488, 12111712, 22802028, 120779959, 337801784, 4361743860, 11425028900, 142573286216, 429917269469, 4138239848098, 16171519847028, 214845911686065

Offset: 1

Author

Jud McCranie

Keywords

• nonn
• nice

Comments

Similar to A036440, but not requiring 1 to be first and n to be last.

Examples

			a(5)=2 since 1,4,3,2,5 and 3,4,1,2,5 have the property that the sum of each adjacent pair is prime.
		

Crossrefs

Cf. A036440, A051237.

Formula

For n>1, a(n) = A103839(n)/2.

Extensions

More terms from Max Alekseyev, Jan 04 2008

a(25)-a(28) from Giovanni Resta, Apr 01 2014