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 A180514 Numbers starting with 1 such that the sum of any two distinct elements has an even number of distinct prime factors. 4
 1, 5, 9, 13, 35, 39, 286, 290, 381, 385, 866, 4376, 10461, 13506, 19709, 50925, 139046, 144086, 188517, 623114, 6815124, 7226204, 7647853, 8970817, 42716373, 64176516, 189403472, 240240118, 463852538, 520740373 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Numbers starting with 2 : 2, 4, 8, 10, 16, 18, 36, 199, 208, 1131, 1347, 3984, 5751, 7310, 27315, 129313, 134101, 169400, 589570,... Numbers starting with 3 : 3, 7, 11, 15, 33, 41, 47, 65, 101, 203, 4102, 6392, 8507, 18608. LINKS EXAMPLE 866 and 19709 are in the sequence because 19709 + 866 = 20575 = 5^2*823 has 2 prime factors. MATHEMATICA t={1}; k=1; Do[k++; While[! And @@ EvenQ[Length /@ FactorInteger[t+k]], k++]; AppendTo[t, k], {18}]; t CROSSREFS Sequence in context: A089977 A024728 A024950 * A118837 A117828 A117830 Adjacent sequences:  A180511 A180512 A180513 * A180515 A180516 A180517 KEYWORD nonn AUTHOR Michel Lagneau, Jan 21 2011 EXTENSIONS a(20)-a(30) from Donovan Johnson, Jan 25 2011 STATUS approved

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