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A260903
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Numbers n such that (2^(2n+7) * 5^(2n+5) + 740711) / 33 is prime (n > 0).
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5
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OFFSET
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1,1
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COMMENTS
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The numbers that follow the expression in the definition have this form: (12) concatenated n times and prepended to 34567.
Empirical observations: primes alternate with nonprimes. 6th (nonprime) and 7th (prime) terms correspond to probable primes. Up to which term the pattern will hold?
(2^(2*a(n)+7) * 5^(2*a(n)+5) + 740711) has 7 proper divisors.
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LINKS
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EXAMPLE
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11 appears because 121212121212121212121234567 ('12' concatenated 11 times and prepended to '34567') is prime.
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MAPLE
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MATHEMATICA
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Select[Range[500], PrimeQ[(2^(2# + 7) * 5^(2# + 5) + 740711)/33] &] (* or *)
Select[Range[50], DivisorSigma[0, (2^(2# + 7) * 5^(2# + 5) + 740711)] - 1 == 7 &] (* inefficient *)
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PROG
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(Magma) [n: n in [1..250] | IsPrime((2^(2*n+7) * 5^(2*n+5) + 740711) div 33)]; // Vincenzo Librandi, Nov 18 2015
(PARI) is(n)=isprime((2^(2*n+7)*5^(2*n+5) + 740711)/33) \\ Anders Hellström, Nov 18 2015
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CROSSREFS
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KEYWORD
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nonn,base,hard,more
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AUTHOR
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STATUS
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approved
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