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4, 9, 21, 25, 26, 33, 58, 82, 85, 93, 95, 111, 115, 129, 177, 213, 237, 265, 267, 309, 321, 329, 335, 365, 381, 411, 427, 505, 519, 545, 565, 579, 581, 597, 633, 655, 679, 687, 699, 723, 753, 755, 785, 789, 831, 835, 879, 895, 921, 951, 973, 985, 1043, 1047, 1115, 1135
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OFFSET
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1,1
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COMMENTS
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4, 9, and 25 are squares. It appears that all the remaining terms are the products of two distinct primes, usually both odd. (But not all such numbers occur, of course.)
It would be nice to have a characterization of these numbers that is independent of A098550.
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LINKS
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MATHEMATICA
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terms = 56;
max = 15 terms;
f[lst_] := Block[{k = 4}, While[GCD[lst[[-2]], k] == 1 || GCD[lst[[-1]], k] > 1 || MemberQ[lst, k], k++]; Append[lst, k]];
A098550 = Nest[f, {1, 2, 3}, max - 3];
sel = Select[Transpose[{Range[max], A098550}], PrimeQ[#[[2]]]&][[All, 1]]+2;
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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