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A266421
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Numbers n such that (2^(n+7)*5^(n+5) - 204979)/9 is prime (n > 0).
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3
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OFFSET
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1,1
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COMMENTS
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Numbers n such that '21669' appended to n times the digit 4 is prime.
Up to a(9) the terms themselves are primes.
a(1), a(2), a(6), a(9), and (2^(a(1)+7) * 5^(a(1)+5) - 204979)/9 = 4444421669 are also Sophie Germain primes.
a(10) > 50000 (if it exists).
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LINKS
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EXAMPLE
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5 appears because 4444421669 ('4' concatenated 5 times and prepended to '21669') is prime.
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MAPLE
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A266421:=n->`if`(isprime((2^(n+7) * 5^(n+5) - 204979)/9), n, NULL): seq(A266421(n), n=1..5000);
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MATHEMATICA
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Select[ Range[5000], PrimeQ[(2^(# + 7) * 5^(# + 5) - 204979)/9] &]
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PROG
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(Magma)[n: n in[1 .. 1000] | IsPrime((2^(n+7) * 5^(n+5) - 204979) div 9)];
(PARI) is(n)=isprime((2^(n+7) * 5^(n+5) - 204979)/9)
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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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