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A070746
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Numbers k such that k 1's followed by k is a prime.
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10
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OFFSET
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1,2
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COMMENTS
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The number corresponding to a(3) has been certified prime with Primo; those corresponding to a(4), a(5), a(6) are probable primes.
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LINKS
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EXAMPLE
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7 is in the sequence because 11111117 (seven 1's followed by 7) is prime.
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MATHEMATICA
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Do[ If[ PrimeQ[ FromDigits[ Join[ IntegerDigits[(10^n - 1)/9], IntegerDigits[n]]]], Print[n]], {n, 1, 2000, 2}]
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PROG
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(WinPFGW) Primality testing r(5107)*10^len(5107)+5107 [N-1/N+1, Brillhart-Lehmer-Selfridge] r(5107)*10^len(5107)+5107 is Fermat and Lucas PRP! (79 seconds). - Jason Earls
(PARI) isok(n) = isprime(eval(concat(apply(x->Str(x), vector(n+1, j, if (j>n, n, 1)))))) \\ Michel Marcus, Jun 15 2018
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CROSSREFS
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KEYWORD
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base,nonn,more
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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