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 A065821 a(n) is the first prime ending in exactly n 1's. 9
 31, 11, 2111, 101111, 311111, 29111111, 61111111, 1711111111, 14111111111, 31111111111, 311111111111, 2111111111111, 31111111111111, 3511111111111111, 5111111111111111, 101111111111111111, 3511111111111111111 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS Harry J. Smith, Table of n, a(n) for n = 1..100 EXAMPLE 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. MATHEMATICA 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 *)_ PROG (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 CROSSREFS Cf. A037055, A065584, A065580, A065581, A065582. Sequence in context: A040935 A145584 A178561 * A040934 A320431 A107114 Adjacent sequences:  A065818 A065819 A065820 * A065822 A065823 A065824 KEYWORD base,nonn AUTHOR Jonathan Ayres (jonathan.ayres(AT)btinternet.com), Nov 23 2001 EXTENSIONS Edited and extended by Robert G. Wilson v, Jul 04 2003 STATUS approved

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Last modified February 28 05:33 EST 2020. Contains 332321 sequences. (Running on oeis4.)