A141584 a(n) = the closest multiple of A053669(2n) to 2n. A053669(2n) = the smallest prime coprime to 2n.

3, 3, 5, 9, 9, 10, 15, 15, 20, 21, 21, 25, 27, 27, 28, 33, 33, 35, 39, 39, 40, 45, 45, 50, 51, 51, 55, 57, 57, 63, 63, 63, 65, 69, 69, 70, 75, 75, 80, 81, 81, 85, 87, 87, 91, 93, 93, 95, 99, 99, 100, 105, 105, 110, 111, 111, 115, 117, 117, 119, 123, 123, 125, 129, 129, 130

Offset

1,1

Links

Table of n, a(n) for n=1..66.

Example

The smallest prime coprime to 30 is 7. The largest multiple of 7 that is < 30 is 28. The smallest multiple of 7 that is > 30 is 35. 28 is closer to 30 than 35 is, so a(15) = 28.

Mathematica

cm[n_]:=Module[{nn=100, prs, cp, m1, m2}, prs=Prime[Range[PrimePi[2*nn]]]; cp=SelectFirst[prs, CoprimeQ[#, n]&]; m1=cp*Floor[n/cp]; m2=m1+cp; If[n-m1<=m2-n, m1, m2]] (* Harvey P. Dale, Jan 19 2019 *)

Crossrefs

Cf. A053669.

Sequence in context: A079586 A354953 A125960 * A287195 A179437 A136791

Adjacent sequences: A141581 A141582 A141583 * A141585 A141586 A141587

Keyword

nonn

Author

Leroy Quet, Aug 19 2008

Extensions

Extended by Ray Chandler, Jun 22 2009

Status

approved