%I #8 Jul 19 2015 01:06:59
%S 1,9,9,9,21,9,51,21,9,57,301,51,51,33,209,111,87,153,121,87,63,39,77,
%T 27,57,81,129,147,111,21,147,321,69,93,153,621,817,129,81,803,129,153,
%U 451,171,717,801,959,459,187,291,231,533,399,291,289,869,489,171,381,667,21
%N a(1) = 1; a(n) = smallest composite number such that the juxtaposition a(1)a(2)...a(n) is a prime.
%e a(5) = 21 and the number 199921 is a prime.
%t a[1] = 1; a[n_] := a[n] = Block[{k = 3, c = IntegerDigits @ Table[ a[i], {i, n - 1}]}, While[PrimeQ[k] || !PrimeQ[FromDigits @ Flatten @ Append[c, IntegerDigits[k]]], k += 2]; k]; Table[ a[n], {n, 61}] (* _Robert G. Wilson v_, Aug 05 2005 *)
%Y Cf. A033680, A074336, A092528.
%K nonn,base
%O 1,2
%A _Amarnath Murthy_, Mar 26 2002
%E More terms from Antonio G. Astudillo (afg_astudillo(AT)lycos.com), Mar 31 2003
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