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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

Table of n, a(n) for n=1..30.

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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Last modified February 26 11:49 EST 2020. Contains 332279 sequences. (Running on oeis4.)