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%I A090920 #10 Jan 13 2017 00:53:50
%S A090920 11,23,31,41,521,61,71,89,919,101,1123,1213,131,1429,151,1697,17137,
%T A090920 181,191,2081,211,2267,2347,241,251,26183,271,281,29581,3061,311,
%U A090920 32257,331,3469,3571,3637,37223,3877,39157,401,41411,421,431,44221,4591,461,47189
%N A090920 Primes of the form n followed by the least k == 1 (mod n).
%C A090920 Conjecture: For n > 1, if a(n) = n concatenated with k then k < n^2.
%H A090920 Robert Israel, <a href="/A090920/b090920.txt">Table of n, a(n) for n = 1..10000</a>
%e A090920 a(5) = 521, as 51,56,511 and 516 are all composite.
%p A090920 f:= proc(n) local k,p;
%p A090920    for k from 1 by n do
%p A090920       p:= n*10^(1+ilog10(k))+k;
%p A090920       if isprime(p) then return p fi
%p A090920    od
%p A090920 end proc:
%p A090920 map(f, [$1..100]); # _Robert Israel_, Jan 13 2017
%o A090920 (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
%Y A090920 Cf. A068695.
%K A090920 base,nonn
%O A090920 1,1
%A A090920 _Amarnath Murthy_, Dec 16 2003
%E A090920 More terms from _David Wasserman_, Feb 14 2006

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