A180514 Numbers starting with 1 such that the sum of any two distinct elements has an even number of distinct prime factors.

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

Offset: 1

Author

Michel Lagneau, Jan 21 2011

Keywords

• nonn

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.

Examples

			866 and 19709 are in the sequence because 19709 + 866 = 20575 = 5^2*823 has 2 prime factors.
		

Programs

Mathematica

t={1}; k=1; Do[k++; While[! And @@ EvenQ[Length /@ FactorInteger[t+k]], k++]; AppendTo[t, k], {18}]; t

Extensions

a(20)-a(30) from Donovan Johnson, Jan 25 2011