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A120729
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Smallest integer k>0 such that k*10^n + 1 is a semiprime.
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0
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3, 2, 2, 5, 1, 1, 1, 1, 1, 2, 4, 2, 3, 7, 4, 3, 6, 6, 4, 1, 2, 4, 13, 2, 4, 3, 7, 21, 6, 9, 3, 1, 5, 4, 16, 19, 28, 19, 9, 3
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OFFSET
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0,1
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COMMENTS
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The corresponding semiprimes are 4, 21, 201, 5001, 10001, 100001, 100001, 10000001, 2000000001, 40000000001, ... Semiprime analog of A121172.
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LINKS
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FORMULA
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Smallest integer k>0 such that k*10^n + 1 is in A001358.
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EXAMPLE
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a(0) = 3 because 3*10^0 + 1 = 4 = 2^2 is a semiprime.
a(1) = 2 because 2*10^1 + 1 = 21 = 3*7 is a semiprime.
a(2) = 2 because 2*10^2 + 1 = 201 = 3*67 is a semiprime.
a(3) = 5 because 5*10^3 + 1 = 5001 = 3*1667 is a semiprime.
a(4) = 1 because 1*10^4 + 1 = 10001 = 73*137 is a semiprime.
a(5) = 1 because 1*10^5 + 1 = 100001 = 11*9091 is a semiprime.
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MATHEMATICA
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sik[n_]:=Module[{k=1, c=10^n}, While[PrimeOmega[k*c+1]!=2, k++]; k]; Array[sik, 40, 0] (* Harvey P. Dale, Aug 20 2012 *)
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CROSSREFS
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KEYWORD
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easy,nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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