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 A030665 Smallest nontrivial extension of n which is prime. 9
 11, 23, 31, 41, 53, 61, 71, 83, 97, 101, 113, 127, 131, 149, 151, 163, 173, 181, 191, 2003, 211, 223, 233, 241, 251, 263, 271, 281, 293, 307, 311, 3203, 331, 347, 353, 367, 373, 383, 397, 401, 419, 421, 431, 443, 457, 461, 479, 487, 491, 503, 5101 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS Chai Wah Wu, Table of n, a(n) for n = 1..10000 [T. D. Noe computed the first 1000 terms] Chai Wah Wu, On a conjecture regarding primality of numbers constructed from prepending and appending identical digits, arXiv:1503.08883 [math.NT], 2015. EXAMPLE For n = 1, we could append 1, 3, 7, 9, 01, etc., to make a prime, but 1 gives the smallest of these, 11, so a(1) = 11. For n = 2, although 2 is already prime, the definition requires an appending at least one digit. 1 doesn't work because 21 = 3 * 7, but 3 does because 23 is prime. Hence a(2) = 23. MAPLE f:= proc(n) local x0, d, r, y;    for d from 1 do      x0:= n*10^d;      for r from 1 to 10^d-1 by 2 do        if isprime(x0+r) then           return(x0+r)        fi      od    od end proc: seq(f(n), n=1..100); # Robert Israel, Dec 23 2014 MATHEMATICA A030665[n_] := Module[{d = 10, nd = 10 * n}, While[True, x = NextPrime[nd]; If[x < nd + d, Return[x]]; d *= 10; nd *= 10]]; Array[A030665, 100] (* Jean-François Alcover, Oct 19 2016, translated from Chai Wah Wu's Python code *) PROG (Python) from sympy import nextprime def A030665(n):     d, nd = 10, 10*n     while True:         x = nextprime(nd)         if x < nd+d:             return int(x)         d *= 10         nd *= 10 # Chai Wah Wu, May 24 2016 CROSSREFS Cf. A018800, A077501. Sequence in context: A038904 A179630 A060399 * A077501 A030675 A110786 Adjacent sequences:  A030662 A030663 A030664 * A030666 A030667 A030668 KEYWORD nonn,base,nice AUTHOR EXTENSIONS Corrected by Ray Chandler, Aug 11 2003 STATUS approved

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Last modified April 5 06:13 EDT 2020. Contains 333238 sequences. (Running on oeis4.)