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A211384
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a(1) = 1, a(2) = 3; for n>2, a(n) = smallest number > a(n-1) such that a(n) is divisible by a(d) for all divisors d of n.
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5
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1, 3, 4, 6, 7, 12, 13, 18, 20, 21, 22, 24, 25, 39, 56, 72, 73, 120, 121, 126, 156, 198, 199, 216, 217, 225, 240, 312, 313, 336, 337, 360, 396, 438, 455, 480, 481, 726, 800, 882, 883, 936, 937, 990, 1120, 1194, 1195, 1296, 1300, 1302, 1460, 1800, 1801, 1920
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OFFSET
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1,2
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COMMENTS
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Conjecture: 10 and 25 are the only composite numbers n for which a(n) = a(n-1) + 1. - J. Lowell, Oct 03 2020
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LINKS
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EXAMPLE
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a(6) = 12 is divisible by a(1) = 1, a(2) = 3, a(3) = 4.
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MAPLE
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a:= proc(n) a(n):= `if`(n<3, 2*n-1, (h-> ceil((a(n-1)+1)/h)*h)
(ilcm(map(a, numtheory[divisors](n) minus {1, n})[]))) end:
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MATHEMATICA
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a[1] = 1; a[2] = 3; a[n_] := a[n] = (Ceiling[(a[n-1]+1)/#]*#&)[LCM @@ Map[a, Most[Divisors[n]]]]; Table[a[n], {n, 1, 60}] (* Jean-François Alcover, Mar 27 2017, after Alois P. Heinz *)
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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