A155487 Difference between n-th composite number and twice its least prime factor.

0, 2, 4, 3, 6, 8, 10, 9, 12, 14, 16, 15, 18, 20, 15, 22, 21, 24, 26, 28, 27, 30, 25, 32, 34, 33, 36, 38, 40, 39, 42, 44, 35, 46, 45, 48, 50, 45, 52, 51, 54, 56, 58, 57, 60, 55, 62, 64, 63, 66, 68, 70, 69, 72, 63, 74, 76, 75, 78, 80, 75, 82, 81, 84, 86, 77, 88, 87, 90, 85, 92, 94

Offset

1,2

Links

Table of n, a(n) for n=1..72.

Example

1st composite is  4 = 2^2; lpf = 2, so a(1) =  4 - 2*2 = 0;
2nd composite is  6 = 2*3; lpf = 2, so a(2) =  6 - 2*2 = 2;
3rd composite is  8 = 2^3; lpf = 2, so a(3) =  8 - 2*2 = 4;
4th composite is  9 = 3^2; lpf = 3, so a(4) =  9 - 2*3 = 3;
5th composite is 10 = 2*5; lpf = 2, so a(5) = 10 - 2*2 = 6.

Mathematica

#-2*FactorInteger[#][[1, 1]]&/@Select[Range[200], CompositeQ] (* Harvey P. Dale, Mar 29 2020 *)

Crossrefs

Cf. A002808.

Sequence in context: A120241 A352714 A355435 * A356430 A128555 A039874

Adjacent sequences: A155484 A155485 A155486 * A155488 A155489 A155490

Keyword

nonn,base

Author

Juri-Stepan Gerasimov, Jan 23 2009

Extensions

Entries checked by R. J. Mathar, May 10 2010

Status

approved