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 A090920 Primes of the form n followed by the least k == 1 (mod n). 1
 11, 23, 31, 41, 521, 61, 71, 89, 919, 101, 1123, 1213, 131, 1429, 151, 1697, 17137, 181, 191, 2081, 211, 2267, 2347, 241, 251, 26183, 271, 281, 29581, 3061, 311, 32257, 331, 3469, 3571, 3637, 37223, 3877, 39157, 401, 41411, 421, 431, 44221, 4591, 461, 47189 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Conjecture: For n > 1, if a(n) = n concatenated with k then k < n^2. LINKS Robert Israel, Table of n, a(n) for n = 1..10000 EXAMPLE a(5) = 521, as 51,56,511 and 516 are all composite. MAPLE f:= proc(n) local k, p; for k from 1 by n do p:= n*10^(1+ilog10(k))+k; if isprime(p) then return p fi od end proc: map(f, [\$1..100]); # Robert Israel, Jan 13 2017 PROG (PARI) a(n)=my(t); forstep(k=1, oo, n, if(isprime(t=10^#digits(k)*n+k), return(t))) \\ Charles R Greathouse IV, Jan 13 2017 CROSSREFS Cf. A068695. Sequence in context: A030675 A110786 A059642 * A344175 A126330 A090506 Adjacent sequences: A090917 A090918 A090919 * A090921 A090922 A090923 KEYWORD base,nonn AUTHOR Amarnath Murthy, Dec 16 2003 EXTENSIONS More terms from David Wasserman, Feb 14 2006 STATUS approved

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Last modified March 24 12:49 EDT 2023. Contains 361479 sequences. (Running on oeis4.)