A126590 Multiples of 3 or 5 but not both.

3, 5, 6, 9, 10, 12, 18, 20, 21, 24, 25, 27, 33, 35, 36, 39, 40, 42, 48, 50, 51, 54, 55, 57, 63, 65, 66, 69, 70, 72, 78, 80, 81, 84, 85, 87, 93, 95, 96, 99, 100, 102, 108, 110, 111, 114, 115, 117, 123, 125, 126, 129, 130, 132, 138, 140, 141, 144, 145, 147, 153, 155, 156

Offset: 1

Author

Zak Seidov, Mar 13 2007

Keywords

• nonn
• easy

Comments

Numbers n such that (n mod 3)*(n mod 5) = 0 and n mod 15 > 0.

Examples

			3, 5, 6, 9, 10, 12, (not 15), 18, 20, 21, 24, 25, 27, (not 30), 33, etc.
		

Links

• Vincenzo Librandi, Table of n, a(n) for n = 1..1000
• Index entries for linear recurrences with constant coefficients, signature (1,0,0,0,0,1,-1).

Crossrefs

Cf. A126073, A126592.

Programs

Magma

I:=[3,5,6,9,10,12,18]; [n le 7 select I[n] else Self(n-1) + Self(n-6) - Self(n-7): n in [1..70]]; // G. C. Greubel, Mar 06 2018

Mathematica

s={}; Do[m3=Mod[n,3]; m5=Mod[n,5]; m15=Mod[n,15]; If[m3*m5==0&&m15>0,AppendTo[s,n]],{n,200}]; s

PARI

is(n)=isprime(gcd(n,15)) \\ Charles R Greathouse IV, Oct 11 2013

Formula

a(n) = a(n-1) + a(n-6) - a(n-7). - Charles R Greathouse IV, Oct 11 2013