OFFSET
1,5
COMMENTS
Not multiplicative: a(21) <> a(3)*a(7), for example. - R. J. Mathar, Sep 15 2015
LINKS
Nick Hobson, Table of n, a(n) for n = 1..10000
Michael Gilleland, Some Self-Similar Integer Sequences.
R. A. Smith and M. V. Subbarao, The average number of divisors in an arithmetic progression, Canadian Mathematical Bulletin, Vol. 24, No. 1 (1981), pp. 37-41.
FORMULA
G.f.: Sum_{n>0} x^n/(1-x^(4n)) = Sum_{n>=0} x^(4n+1)/(1-x^(4n+1)).
Sum_{k=1..n} a(k) = n*log(n)/4 + c*n + O(n^(1/3)*log(n)), where c = gamma(1,4) - (1 - gamma)/4 = A256778 - (1 - A001620)/4 = 0.604593... (Smith and Subbarao, 1981). - Amiram Eldar, Nov 25 2023
MAPLE
d:=proc(r, m, n) local i, t1; t1:=0; for i from 1 to n do if n mod i = 0 and i-r mod m = 0 then t1:=t1+1; fi; od: t1; end; # no. of divisors i of n with i == r mod m
A001826 := proc(n)
add(`if`(modp(d, 4)=1, 1, 0), d=numtheory[divisors](n)) ;
end proc: # R. J. Mathar, Sep 15 2015
MATHEMATICA
a[n_] := Count[Divisors[n], d_ /; Mod[d, 4] == 1]; Table[a[n], {n, 1, 105}] (* Jean-François Alcover, Nov 26 2013 *)
a[n_] := DivisorSum[n, 1 &, Mod[#, 4] == 1 &]; Array[a, 100] (* Amiram Eldar, Nov 25 2023 *)
PROG
(PARI) a(n)=if(n<1, 0, sumdiv(n, d, d%4==1))
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
EXTENSIONS
Better definition from Michael Somos, Apr 26 2004
STATUS
approved