A090920 - OEIS

%I #10 Jan 13 2017 00:53:50

%S 11,23,31,41,521,61,71,89,919,101,1123,1213,131,1429,151,1697,17137,

%T 181,191,2081,211,2267,2347,241,251,26183,271,281,29581,3061,311,

%U 32257,331,3469,3571,3637,37223,3877,39157,401,41411,421,431,44221,4591,461,47189

%N Primes of the form n followed by the least k == 1 (mod n).

%C Conjecture: For n > 1, if a(n) = n concatenated with k then k < n^2.

%H Robert Israel, <a href="/A090920/b090920.txt">Table of n, a(n) for n = 1..10000</a>

%e a(5) = 521, as 51,56,511 and 516 are all composite.

%p f:= proc(n) local k,p;

%p    for k from 1 by n do

%p       p:= n*10^(1+ilog10(k))+k;

%p       if isprime(p) then return p fi

%p    od

%p end proc:

%p map(f, [$1..100]); # _Robert Israel_, Jan 13 2017

%o (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 Cf. A068695.

%K base,nonn

%O 1,1

%A _Amarnath Murthy_, Dec 16 2003

%E More terms from _David Wasserman_, Feb 14 2006