OFFSET
1,1
LINKS
Ken Davis, Status of Search for Multifactorial Primes.
Ken Davis, Results for n!2-1.
FORMULA
a(n) = A007749(n+1)/2. - Alexander Adamchuk, Sep 23 2006
EXAMPLE
a(1)=2 because 2!*2^2 - 1 = 7 is prime
a(2)=3 because 3!*2^3 - 1 = 47 is prime
MATHEMATICA
For[n=1, n<1000, n++, If[PrimeQ[2^n*n!-1], Print[n]]] (Steinerberger)
PROG
(PARI) f(n)=n!*2^n -1; for (i=1, 363, if(isprime(f(i)), print(i)))
CROSSREFS
KEYWORD
hard,more,nonn
AUTHOR
Mohammed Bouayoun (bouyao(AT)wanadoo.fr), Mar 02 2004
EXTENSIONS
a(12)-a(14) from Stefan Steinerberger, Feb 05 2006
a(15) from Mohammed Bouayoun (Mohammed.Bouayoun(AT)yahoo.fr), Apr 13 2006
More terms from Alexander Adamchuk, Sep 23 2006
Corrected and extended by Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 03 2008
Terms a(22)..a(24) (using A007749) from Joerg Arndt, Apr 22 2016
STATUS
approved