A212321 Minimal m >= 1 such that floor((2*n-1)!!/m) + 2 is prime.

1, 1, 1, 1, 1, 3, 9, 5, 5, 3, 1, 1, 7, 1, 13, 11, 15, 21, 23, 15, 17, 5, 25, 25, 7, 85, 9, 25, 1, 21, 9, 13, 17, 5, 49, 39, 45, 1, 43, 51, 145, 17, 143, 85, 19, 81, 63, 43, 47, 77, 23, 1, 3, 21, 17, 47, 37, 3, 7, 67, 23, 23, 25, 45, 81, 5, 39, 145, 71, 21, 41

Offset: 1

Author

Vladimir Shevelev, Feb 14 2013

Keywords

• nonn

Links

• Alois P. Heinz, Table of n, a(n) for n = 1..500

Crossrefs

Cf. A212281, A212282.

Programs

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=1..50);  # Alois P. Heinz, Feb 18 2013

Mathematica

a[n_] := Module[{m}, For[m = 1, True, m++, If[PrimeQ[Floor[(2n-1)!!/m]+2], Return[m]]]];
Array[a, 100] (* Jean-François Alcover, Oct 28 2020 *)

Extensions

More terms from Alois P. Heinz, Feb 18 2013