%I
%S 1,3,7,19,21,31,33,39,41,49,51,59,61,71,79,93,107,119,121,123,139,147,
%T 159,169,171,177,179,181,191,197,219,233,247,253,257,263,273,281,293,
%U 343,351,359,391,417,427,439,441,449,451,459,461,489,503,517,529,531
%N a(n) = smallest m > a(n1) such that concatenation nm is a prime.
%C Up to a(10^6), a(n) =~ 100000 + 24n + 10^(6*n), which is almost a straight line.  _Robert G. Wilson v_
%e a(5) = 21 because the first prime after a(4) = 419 beginning with 5 is 521.
%t a[1] = 1; a[1] = 1; a[n_] := a[n] = Block[{k = IntegerDigits[n], l = a[n  1] + 2}, While[ !PrimeQ[ FromDigits[ Join[k, IntegerDigits[l]]]], l += 2]; l]; Table[ a[n], {n, 1, 55}]
%K base,nonn
%O 1,2
%A _Amarnath Murthy_, Nov 20 2003
%E Corrected and extended by _Robert G. Wilson v_, Dec 05 2003
