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 A089762 a(1) = 1, then nonzero digits ( 1 to 9) such that every n-th concatenation is prime if n is prime else it is composite. The previous digits are so chosen that a single digit with prime index gives a prime. 0
 1, 1, 3, 1, 1, 2, 1, 1, 1, 2, 9, 1, 1, 1, 1, 4, 3, 4, 3, 1, 1, 1, 3, 1, 1, 1, 1, 4, 1, 2, 3, 1, 1, 1, 2, 5, 9, 1, 1, 6, 9, 9, 1, 1, 2, 2, 7, 1, 1, 1, 2, 3, 9, 1, 1, 1, 1, 7, 1, 7, 9, 1, 1, 1, 7, 5, 3, 1, 2, 8, 3, 3, 7, 1, 1, 1, 3, 4, 7, 1, 1, 5, 9, 1, 1, 1, 1, 7, 3, 1, 1, 1, 1, 1, 1, 4, 9, 1, 1, 5, 7, 9, 3, 4, 3 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS This is the lexicographically least sequence that fits the rule through 114 digits. There is no guarantee that it can be extended indefinitely. - David Wasserman, Oct 06 2005 LINKS EXAMPLE The first 11 partial concatenations are 1,11,113,1131,11311,113112,1131121,11311211,113112111,1131121112,11311211129. The 2nd, 3rd, 5th 7th and 11th terms are primes. rest are composite. PROG (PARI) num = 111; n = 3; while (n < 115, isp = isprime(n); while (num%10 && isprime(num) != isp, num++); if (num%10, n++; num = 10*num + 1, num = (num - 1)\10 + 1; n--)); num\10; (David Wasserman, Sep 20 2005) CROSSREFS Sequence in context: A285770 A174820 A099501 * A257567 A189965 A258820 Adjacent sequences:  A089759 A089760 A089761 * A089763 A089764 A089765 KEYWORD nonn,base AUTHOR Amarnath Murthy, Nov 22 2003 EXTENSIONS More terms from David Wasserman, Oct 06 2005 Edited by Charles R Greathouse IV, Apr 29 2010 STATUS approved

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