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A186429
Numbers n such that sum_{i=1..n} d(i)^2 is a square c^2, where d(i) is the number of divisors of i.
1
1, 3, 28, 85, 151, 302, 864, 3197, 4780, 5438, 5815, 6413, 7769, 7985, 11062, 16672, 24805, 27379, 35435, 41718, 45047, 58383, 69835, 70235, 76305, 91178, 95963, 97763, 103173, 111375, 118498, 138405, 158200, 170333, 184132, 191707, 201693
OFFSET
1,2
COMMENTS
c is from (1,3,21,49,74,121,249,590,765,...)
LINKS
Charles R Greathouse IV, Table of n, a(n) for n = 1..1000
EXAMPLE
For n=3 we have (1*1+2*2+2*2)= 3*3 so n=3 belongs to the sequence.
MATHEMATICA
Position[Accumulate[DivisorSigma[0, Range[210000]]^2], _?(IntegerQ[ Sqrt[ #]]&)]//Flatten (* Harvey P. Dale, Jul 17 2016 *)
PROG
(PARI) s=0; for(n=1, 1e7, if(issquare(s+=numdiv(n)^2), print1(n", "))) \\ Charles R Greathouse IV, Feb 21 2011
CROSSREFS
Cf. A000005.
Sequence in context: A116984 A229055 A061793 * A165393 A107651 A239057
KEYWORD
nonn
AUTHOR
Ctibor O. Zizka, Feb 21 2011
STATUS
approved