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A228149
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a(n) is the smallest n-digit prime which remains prime whenever a 7 is inserted, appended or prepended, or 0 if no such prime exists.
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2
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3, 19, 433, 1471, 13177, 175687, 1740787, 10871017, 137725897, 1670181787, 12845952529, 117777772309, 1008078796477, 11777787780727
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OFFSET
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1,1
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COMMENTS
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LINKS
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EXAMPLE
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a(3) = 433 is the smallest 3-digit prime which yields 4 primes by inserting a 7 in every possible position: 7433, 4733, 4373, 4337.
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MATHEMATICA
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pp[n_, k_] := Catch[Block[{d = IntegerDigits@n}, Do[If[! PrimeQ[ FromDigits[ Insert[d, k, i]]], Throw[False]], {i, 1+Length@d}]; True]]; a[n_] := Catch[ Block[{p = NextPrime[10^(n-1)]}, While[p < 10^n, If[pp[p, 7], Throw@p, p = NextPrime@p]]; 0]]; a /@ Range[8]
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CROSSREFS
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KEYWORD
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nonn,base,more
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AUTHOR
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STATUS
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approved
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