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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 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 LINKS 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 Cf. A033680, A074336, A092528. Sequence in context: A072563 A206011 A192984 * A160761 A082049 A282242 Adjacent sequences:  A069599 A069600 A069601 * A069603 A069604 A069605 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

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Last modified May 20 10:33 EDT 2022. Contains 353871 sequences. (Running on oeis4.)