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A065083
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The least k such that precisely n near-repunit primes can be formed from (10^k-1)/9 by changing one digit from 1 to 0.
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5
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1, 3, 8, 24, 20, 12, 488, 42, 162, 4848, 642, 1682
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OFFSET
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0,2
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COMMENTS
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LINKS
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EXAMPLE
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a(5) = 12 because R_12 = (10^12 -1)/9 = 111111111111 and from this number, by changing just one digit from 1 to 0, out of the eleven candidates, 111111111101, 111111110111, 111111011111, 111011111111 and 101111111111 are primes.
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MATHEMATICA
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a = Table[0, {10} ]; Do[p = IntegerDigits[ (10^n - 1)/9]; c = 0; Do[ If[ q = FromDigits[ ReplacePart[p, 0, i]]; PrimeQ[q], c++ ], {i, 2, n} ]; If[ a[[c + 1]] == 0, a[[c + 1]] = n], {n, 1, 400} ]; a
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PROG
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(PARI) a(n)=my(k=1); while(sum(i=1, k-2, ispseudoprime(10^k\9-10^i)) != n, k++); k \\ Charles R Greathouse IV, May 01 2012
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CROSSREFS
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KEYWORD
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more,nonn,base
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AUTHOR
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EXTENSIONS
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a(10)-a(11) from comments and verified by Robert Price, Nov 04 2023
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STATUS
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approved
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