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 A000423 a(n) is smallest number > a(n-1) of form a(i)*a(j), i < j < n. 4
 2, 3, 6, 12, 18, 24, 36, 48, 54, 72, 96, 108, 144, 162, 192, 216, 288, 324, 384, 432, 486, 576, 648, 768, 864, 972, 1152, 1296, 1458, 1536, 1728, 1944, 2304, 2592, 2916, 3072, 3456, 3888, 4374, 4608, 5184, 5832, 6144, 6912, 7776, 8748, 9216, 10368, 11664 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Sequence contains 2, 3 and all numbers of form 2^a*3^b where a >= 1 and b >= 1. - David W. Wilson, Aug 15 1996 Main entry for this sequence is A033845, which is this sequence starting at 6. - Charles R Greathouse IV, Feb 27 2012 REFERENCES Amarnath Murthy, The sum of the reciprocals of the Smarandache multiplicative sequence, (to be published in Smarandache Notions Journal). F. Smarandache, "Properties of the Numbers", University of Craiova Archives, 1975; Arizona State University Special Collections, Tempe, AZ M. Myers, Smarandache Multiplicative Numbers, in Memorables 1998, Bristol Banner Books, Bristol, p. 37, 1998. LINKS F. Smarandache, Sequences of Numbers Involved in Unsolved Problems. FORMULA Sum_{n>=1} 1/a(n) = 4/3. - Amiram Eldar, Jul 31 2022 MATHEMATICA a[1] = 2; a[2] = 3; a[n_] := a[n] = For[k = a[n - 1] + 1, True, k++, If[ AnyTrue[Table[a[i] a[j], {i, 1, n-2}, {j, i+1, n-1}] // Flatten, # == k& ], Return[k]]]; Table[an = a[n]; Print[an]; an, {n, 1, 50}] (* Jean-François Alcover, Feb 08 2016 *) CROSSREFS Subsequence of A003586 (3-smooth numbers). A007335 and A033845 are subsequences. Sequence in context: A280681 A328899 A093687 * A007335 A309311 A361693 Adjacent sequences: A000420 A000421 A000422 * A000424 A000425 A000426 KEYWORD nonn,easy AUTHOR R. Muller EXTENSIONS More terms from David W. Wilson, Aug 15 1996 STATUS approved

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Last modified March 31 16:08 EDT 2023. Contains 361668 sequences. (Running on oeis4.)