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A069602
a(1) = 1; a(n) = smallest composite number such that the juxtaposition a(1)a(2)...a(n) is a prime.
9
1, 9, 9, 9, 21, 9, 51, 21, 9, 57, 301, 51, 51, 33, 209, 111, 87, 153, 121, 87, 63, 39, 77, 27, 57, 81, 129, 147, 111, 21, 147, 321, 69, 93, 153, 621, 817, 129, 81, 803, 129, 153, 451, 171, 717, 801, 959, 459, 187, 291, 231, 533, 399, 291, 289, 869, 489, 171, 381, 667, 21
OFFSET
1,2
EXAMPLE
a(5) = 21 and the number 199921 is a prime.
MATHEMATICA
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 *)
CROSSREFS
KEYWORD
nonn,base
AUTHOR
Amarnath Murthy, Mar 26 2002
EXTENSIONS
More terms from Antonio G. Astudillo (afg_astudillo(AT)lycos.com), Mar 31 2003
STATUS
approved