A217651 Primes of the form 2*k!!! - 1.

3, 5, 7, 19, 1759, 192684799, 14285134079, 145284339199, 509122178611199, 53165268316159999, 253515360530431999, 89312661819089868632723554303999999, 25863781053916181957690694149957243371519999999

Offset: 1

Author

Michel Lagneau, Oct 09 2012

Keywords

• nonn

Comments

k!!! is a triple factorial number (see the definition in A007661).

The corresponding k are in A217650.

Examples

			19 is in the sequence because k = 5 => 2*5!!! - 1 = 2*10 - 1 = 19 is prime.
		

Crossrefs

Cf. A007661, A217650, A217648.

Programs

Maple

A:= n -> mul(k, k = select(k -> k mod 3 = n mod 3, [$1 .. n])): for p from 0 to 200 do:if type(2*A(p)-1,prime)=true then printf(`%d, `,2*A(p)-1):else fi:od:

Mathematica

multiFactorial[n_, k_] := If[n < 1, 1, If[n < k + 1, n, n*multiFactorial[n - k, k]]];
Select[Table[2*multiFactorial[n, 3] - 1, {n, 0, 60}], PrimeQ]