A166080 Nonprimes of the form (k^2+1)/2.

1, 25, 85, 145, 221, 265, 365, 481, 545, 685, 841, 925, 1105, 1405, 1513, 1625, 1985, 2245, 2665, 2813, 2965, 3281, 3445, 3785, 3961, 4141, 4325, 4705, 4901, 5305, 5513, 5725, 5941, 6161, 6385, 6613, 6845, 7081, 7565, 7813, 8065, 8321, 8845, 9113, 9385

Offset: 1

Author

Juri-Stepan Gerasimov, Oct 06 2009

Keywords

• nonn
• easy

Comments

Or, 1 together with composite numbers of the form i^2+(i+1)^2. See A012132. - N. J. A. Sloane, Feb 29 2020

A166080 U A027862 = A001844.

Examples

			a(1)=(1^2+1)/2=1; a(2)=(7^2+1)/2=25.
		

Links

• Harvey P. Dale, Table of n, a(n) for n = 1..1000

Crossrefs

Cf. A001844, A027862, A012132, A141468.

Programs

Mathematica

Select[(Range[0,150]^2+1)/2,IntegerQ[#]&&!PrimeQ[#]&] (* Harvey P. Dale, Aug 09 2025 *)

Formula

a(n) = 2n^2 + O(n^2/log n). - Charles R Greathouse IV, Mar 21 2014

Extensions

Replaced 6261 by 6161 - R. J. Mathar, Oct 07 2009