A074170 Start with 1, add the next number if one gets a prime then add the next number else subtract the next...

1, 3, 6, 2, 7, 13, 20, 12, 3, 13, 24, 12, -1, -15, -30, -46, -63, -81, -100, -120, -141, -163, -140, -164, -189, -215, -242, -270, -299, -329, -360, -392, -425, -459, -494, -530, -567, -605, -644, -684, -725, -767, -810, -854, -899, -945, -992, -1040, -1089, -1139, -1190, -1242, -1295, -1349, -1404, -1460

Offset: 1

Author

Amarnath Murthy, Aug 30 2002

Keywords

• easy
• sign

Comments

Note that a(22) = -163 is the last prime generated by this sequence. All subsequent terms are composite and equal (16-n)(n+17)/2.

Examples

			a(1) = 1, a(2) = 1+2 =3 is a prime hence a(3) = 3 +3 = 6 which is composite hence a(4) = 6-4 = 2 etc.
		

Crossrefs

Cf. A074171.

Programs

Mathematica

a=3; Join[{1, 3}, Table[If[PrimeQ[a], a=a+n, a=a-n], {n, 3, 60}]]

Formula

a(n) = -(n-16)(n+17)/2 for n > 22

Extensions

Corrected and extended by Jason Earls, Sep 01 2002

Corrected by T. D. Noe, Oct 04 2004