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A102500
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Values of n for which the concatenation 7nn7 (from sequence A100897) are primes.
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3
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15, 18, 19, 34, 36, 39, 48, 52, 57, 60, 67, 75, 79, 81, 85, 96, 97, 1005, 1020, 1030, 1047, 1054, 1062, 1065, 1074, 1084, 1086, 1090, 1096, 1108, 1128, 1152, 1153, 1164, 1171, 1173, 1179, 1182, 1185, 1189, 1213, 1215, 1219, 1228, 1231, 1233, 1257, 1269, 1279
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OFFSET
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1,1
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LINKS
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EXAMPLE
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The number 715157 is prime and corresponds to n=15.
The number 7100510057 is prime and corresponds to n=1005.
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MATHEMATICA
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fQ[n_] := Block[{id = IntegerDigits[n]}, PrimeQ[ FromDigits[ Join[{7}, id, id, {7}]]]]; Select[ Range[1290], fQ[ # ] &] (* Robert G. Wilson v, Jan 14 2004 *)
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CROSSREFS
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The full sequence of integers of the form 7nn7 is A100897. The primes that correspond to these values of n are sequence A102501.
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KEYWORD
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easy,nonn,base
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AUTHOR
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Mark Hudson (mrmarkhudson(AT)hotmail.com), Jan 13 2005
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EXTENSIONS
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STATUS
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approved
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