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A124989
Primes of the form 10*k + 9 generated recursively. Initial prime is 19. General term is a(n) = Min_{p is prime; p divides 100*Q^2 - 5; p == 9 (mod 10)}, where Q is the product of previous terms in the sequence.
2
19, 7219, 462739, 509, 129229, 295380580489, 9653956849, 149, 110212292237172705230749846071050188009093377022084806290042881946231583507557298889, 157881589, 60397967745386189, 1429, 79
OFFSET
1,1
COMMENTS
100Q^2-5 always has a prime divisor congruent to 9 modulo 10.
LINKS
EXAMPLE
a(3) = 462739 is the smallest prime divisor congruent to 9 mod 10 of 100Q^2-5 = 1881313992095 = 5 * 462739 * 813121, where Q = 19 * 7219.
MATHEMATICA
a={19}; q=1;
For[n=2, n<=6, n++,
q=q*Last[a];
AppendTo[a, Min[Select[FactorInteger[100*q^2-5][[All, 1]], Mod[#, 10]==9&]]];
];
a (* Robert Price, Jul 18 2015 *)
KEYWORD
nonn
AUTHOR
Nick Hobson, Nov 18 2006
STATUS
approved