A116889 - OEIS

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

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OFFSET

0,1

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.

LINKS

Table of n, a(n) for n=0..4.

EXAMPLE

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.

Sequence in context: A323340 A280895 A088266 * A347716 A037847 A037883

Adjacent sequences: A116886 A116887 A116888 * A116890 A116891 A116892

KEYWORD

fini,full,nonn

AUTHOR

Giovanni Resta, Feb 27 2006

EXTENSIONS

Typo in Example fixed by Zak Seidov, Nov 07 2013

STATUS

approved