OFFSET
1,1
COMMENTS
The n-th pentagonal number is (3*n^2-n)/2 = n*(3*n-1)/2.
LINKS
K. D. Bajpai, Table of n, a(n) for n = 1..8500
EXAMPLE
a(2)= 19: n= 4: (3*n^2-n)/2-3= 19, which is prime.
a(6)= 373: n= 16: (3*n^2-n)/2-3= 373, which is prime.
MAPLE
KD:= proc() local a, b; a:= (3*n^2-n)/2; b:=a-3; if isprime(b) then RETURN (b): fi; end: seq(KD(), n=1..500);
MATHEMATICA
Select[Table[(n(3n-1))/2-3, {n, 2, 200}], PrimeQ] (* Harvey P. Dale, Jul 11 2015 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
K. D. Bajpai, Nov 25 2013
STATUS
approved