OFFSET
1,3
COMMENTS
Sum of numbers m<=n such that mod(m,3)*mod(m,5)=0 and mod(m,15)>0.
First differences (fd) are
0,3,0,5,6,0,0,9,10,0,12,0,0,0,0,
0,18,0,20,21,0,0,24,25,0,27,0,0,0,0,
0,33,0,35,36,0,0,39,40,0,42,0,0,0,0,...
fd(1..15)={0,3,0,5,6,0,0,9,10,0,12,0,0,0,0}; for n>15
fd(n)=fd(n-15)+15 if fd(n-15)>0, fd(n)=0 otherwise.
LINKS
Ray Chandler, Table of n, a(n) for n = 1..1000
Index entries for linear recurrences with constant coefficients, signature (1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 2, -2, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, -1, 1).
FORMULA
an[n,d]=d*Floor[n/d];sn[n,d]=(an[n,d]*(an[n,d] + d))/(2*d); a(n)=sn[n,3]+sn[n,5]-2*sn[n,15].
MATHEMATICA
an[n_, d_]:=d*Floor[n/d]; sn[n_, d_]:=(an[n, d]*(an[n, d] + d))/(2*d); Table[sn[n, 3]+sn[n, 5]-2*sn[n, 15], {n, 1000}]
CROSSREFS
KEYWORD
nonn
AUTHOR
Zak Seidov, Mar 13 2007
STATUS
approved