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A268702
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Largest n digit prime having at least n-1 digits equal to 1.
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6
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7, 71, 911, 8111, 16111, 911111, 1171111, 71111111, 131111111, 1711111111, 31111111111, 311111111111, 5111111111111, 41111111111111, 111151111111111, 5111111111111111, 11111611111111111, 191111111111111111, 2111111111111111111, 11111111611111111111
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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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a(3) = 911 since 111, 211, 311, ..., 811 are all composites but 911 is prime. Also of the ten primes of 3 digits which contain at least 2 ones, {101, 113, 131, 151, 181, 191, 211, 311, 811, 911}, 911 is the greatest.
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MATHEMATICA
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f[n_] := Block[{k = 0, p = {}, r = (10^n - 1)/9, s = Range@ 10 - 2}, While[k < n, AppendTo[p, Select[r + 10^k*s, PrimeQ]]; k++]; p = Max@ Flatten@ p]; Array[f, 20]
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PROG
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(PARI) a(n) = {p = precprime(10^n-1); while (#select(x->x==1, digits(p)) != n-1, p = precprime(p-1)); p; } \\ Michel Marcus, Feb 21 2016
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CROSSREFS
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KEYWORD
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nonn,base
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AUTHOR
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STATUS
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approved
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