OFFSET
1,1
COMMENTS
The next term -- a(8) -- has 675 digits. - Harvey P. Dale, Dec 17 2018
REFERENCES
G. Balzarotti and P. P. Lava, Le sequenze di numeri interi, Hoepli, 2008, p. 159.
EXAMPLE
For n=4, 4! - 4!! + 1 = 4*3*2*1 - 4*2 + 1 = 24 - 8 + 1 = 17, which is prime.
MAPLE
SFACT:= proc(n) local i, j, k; for k from 1 by 1 to n do i:=k; j:=k-2; while j >0 do i:=i*j; j:=j-2; od: if isprime(k!-i+1) then print(k!-i+1); fi; od: end: SFACT(300);
MATHEMATICA
Select[Table[n!-n!!+1, {n, 70}], PrimeQ] (* Harvey P. Dale, Dec 17 2018 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Paolo P. Lava and Giorgio Balzarotti, May 22 2006
STATUS
approved