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A276047
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Numbers k such that 4*10^k + 21 is prime.
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0
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1, 2, 3, 7, 15, 22, 30, 35, 44, 73, 89, 91, 224, 533, 821, 1037, 1338, 1458, 1777, 2046, 2257, 2877, 3047, 3407, 13398, 42766, 55906, 61625, 66653, 123113, 229836, 238163
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OFFSET
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1,2
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COMMENTS
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For k > 1, numbers k such that the digit 4 followed by k-2 occurrences of the digit 0 followed by the digits 21 is prime (see Example section).
a(33) > 3*10^5.
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LINKS
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EXAMPLE
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3 is in this sequence because 4*10^3 + 21 = 4021 is prime.
Initial terms and associated primes:
a(1) = 1, 61;
a(2) = 2, 421;
a(3) = 3, 4021;
a(4) = 7, 40000021;
a(5) = 15, 4000000000000021, etc.
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MATHEMATICA
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Select[Range[0, 100000], PrimeQ[4*10^# + 21] &]
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PROG
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CROSSREFS
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KEYWORD
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nonn,more
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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