A116889 a(n) is the least prime p that remains prime through n iterations of function f(p)=p^2+4.

2, 3, 3, 5087, 306167

Offset: 0

Author

Giovanni Resta, Feb 27 2006

Keywords

• fini
• full
• nonn

Comments

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.

Examples

			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.
		

Crossrefs

Cf. A062324, A116886, A116887, A116888.

Extensions

Typo in Example fixed by Zak Seidov, Nov 07 2013