OFFSET
1,2
COMMENTS
The sequence is periodic with period length 30.
LINKS
G. C. Greubel, Table of n, a(n) for n = 1..10000
V. Shevelev, A recursion for divisor function over divisors belonging to a prescribed finite sequence of positive integers and a solution of the Lahiri problem for divisor function sigma_x(n), arXiv:0903.1743 [math.NT], 2009.
Index entries for linear recurrences with constant coefficients, signature (-2,-2,-1,0,1,2,2,1).
FORMULA
a(n) = -2*a(n-1) -2*a(n-2) -a(n-3) +a(n-5) +2*a(n-6) +2*a(n-7) +a(n-8). R. J. Mathar, Jul 23 2012
G.f.: ( -x*(2+7*x+12*x^2+17*x^3+22*x^4+10*x^6+20*x^5) ) / ( (x-1)*(1+x)*(1+x+x^2)*(x^4+x^3+x^2+x+1) ). - R. J. Mathar, Jul 23 2012
MAPLE
A178144 := proc(n)
local a;
a := 0 ;
for d in {2, 3, 5} do
if (n mod d) = 0 then
a := a+d ;
end if;
end do:
a ;
end proc: # R. J. Mathar, Jul 23 2012
MATHEMATICA
a[n_] := DivisorSum[n, Boole[MatchQ[#, 2|3|5]]*#&];
Array[a, 105] (* Jean-François Alcover, Nov 24 2017 *)
PROG
(PARI) a(n) = sumdiv(n, d, if ((d==2) || (d==3) || (d==5), d)); \\ Michel Marcus, Nov 24 2017
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Vladimir Shevelev, May 21 2010
STATUS
approved