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A116889
a(n) is the least prime p that remains prime through n iterations of function f(p)=p^2+4.
4
2, 3, 3, 5087, 306167
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.
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
KEYWORD
fini,full,nonn
AUTHOR
Giovanni Resta, Feb 27 2006
EXTENSIONS
Typo in Example fixed by Zak Seidov, Nov 07 2013
STATUS
approved