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A065821
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a(n) is the first prime ending in exactly n 1's.
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9
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31, 11, 2111, 101111, 311111, 29111111, 61111111, 1711111111, 14111111111, 31111111111, 311111111111, 2111111111111, 31111111111111, 3511111111111111, 5111111111111111, 101111111111111111, 3511111111111111111
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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(4) = 101111 because 1111=11*101, 21111=3*31*227, 31111=53*587, 41111=7^2*829, 51111=3^4*631, 61111=23*2657, 71111=17*47*89, 81111=3*19*1423, 91111=179*509 so 101111 is the first prime ending in four 1's.
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MATHEMATICA
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pe[n_]:=Module[{k=0, len=IntegerLength[n]}, While[Mod[k, 10]==1||(!PrimeQ[ k*10^len+n]), k++]; k*10^len+n]; pe/@Table[(10^n-1)/9, {n, 20}] (* Harvey P. Dale, Dec 31 2013 *)_
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PROG
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(PARI) { b=0; f=1; for (n=1, 100, b=10*b + 1; f*=10; k=0; while (!isprime(b + k*f), if (k%10, k++, k+=2)); write("b065821.txt", n, " ", b + k*f) ) } \\ Harry J. Smith, Nov 01 2009
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CROSSREFS
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KEYWORD
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base,nonn
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AUTHOR
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Jonathan Ayres (jonathan.ayres(AT)btinternet.com), Nov 23 2001
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EXTENSIONS
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STATUS
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approved
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