OFFSET
1,4
EXAMPLE
a(4) = 2 because a(4-1) = 1 is not prime, so a(4) = a(3) + 1 = 2.
a(5) = 4 because a(5-1) = 2 is prime, so a(5) = a(4) + a(3) + a(2) = 2+1+1 = 4.
a(6) = 5 because a(6-1) = 4 is not prime, so a(6) = a(5) + 1 = 4 + 1 = 5.
a(7) = 11 because a(7-1) = 5 is prime, so a(7) = a(6) + a(5) + a(4) = 5+4+2 = 11.
a(8) = 20 because a(8-1) = 11 is prime, so a(8) = a(7)+a(6)+a(5) = 11+5+4 = 20.
MATHEMATICA
nxt[{a_, b_, c_}]:=If[PrimeQ[c], {b, c, a+b+c}, {b, c, c+1}]; Transpose[NestList[ nxt, {1, 1, 1}, 50]][[1]] (* Harvey P. Dale, Mar 25 2012 *)
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Jonathan Vos Post, Oct 13 2005
EXTENSIONS
Corrected and extended by Harvey P. Dale, Mar 25 2012
STATUS
approved