A212636 Minimal m >= 1 such that floor((2*n - 1)!!/m) - 2 is prime.

1, 1, 3, 3, 8, 1, 1, 1, 5, 7, 19, 7, 5, 15, 7, 17, 5, 3, 9, 11, 63, 9, 5, 1, 53, 27, 51, 11, 3, 11, 13, 15, 17, 35, 1, 17, 21, 13, 139, 61, 3, 13, 1, 7, 147, 23, 123, 47, 41, 35, 11, 39, 69, 21, 123, 29, 27, 49, 3, 9, 37, 171, 57, 1, 31, 37, 5, 61, 27, 31, 53

Offset

3,3

Links

Table of n, a(n) for n=3..73.

Example

(2*6-1)!! = 11!! = 11 * 9 * 7 * 5 * 3 * 1 = 10395. Floor(10395/1) - 2 = 10395 - 2 = 10393 =  19 * 547 is not prime, and floor(10395/2) - 2 = 5197 - 2 = 5195 = 5 * 1039 is not prime, but floor(10395/3) - 2 = 3465 - 2 = 3463 is prime, so a(6) = 3.

Maple

a:= proc(n) local m;
      for m while not isprime(iquo(doublefactorial(2*n-1), m)-2)
      do od; m
    end:
seq(a(n), n=3..70);  #  Alois P. Heinz, Feb 18 2013

Crossrefs

Cf. A212281, A212282, A212321.

Sequence in context: A171543 A079073 A165507 * A282255 A164040 A154178

Adjacent sequences: A212633 A212634 A212635 * A212637 A212638 A212639

Keyword

nonn

Author

Vladimir Shevelev, Feb 14 2013

Extensions

More terms from Alois P. Heinz, Feb 18 2013

Status

approved