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%I #5 Nov 07 2013 02:53:22
%S 2,3,3,5087,306167
%N a(n) is the least prime p that remains prime through n iterations of function f(p)=p^2+4.
%C The sequence is finite, since it can be proved that if p, f(p), f(f(p)), f(f(f(p))) and f(f(f(f(p)))) are all primes, then the next iteration gives a multiple of 13, greater than 13, thus a(k) for k>=5 does not exist.
%e a(0)=2 since f(2)=8 is not prime. a(1)=a(2)=3 since both f(3)=13 and f(f(3))=173 are primes.
%Y Cf. A062324, A116886, A116887, A116888.
%K fini,full,nonn
%O 0,1
%A _Giovanni Resta_, Feb 27 2006
%E Typo in Example fixed by _Zak Seidov_, Nov 07 2013