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%I #18 Mar 01 2022 11:56:50
%S 1,3,28,85,151,302,864,3197,4780,5438,5815,6413,7769,7985,11062,16672,
%T 24805,27379,35435,41718,45047,58383,69835,70235,76305,91178,95963,
%U 97763,103173,111375,118498,138405,158200,170333,184132,191707,201693
%N 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.
%C c is from (1,3,21,49,74,121,249,590,765,...)
%H Charles R Greathouse IV, <a href="/A186429/b186429.txt">Table of n, a(n) for n = 1..1000</a>
%e For n=3 we have (1*1+2*2+2*2)= 3*3 so n=3 belongs to the sequence.
%t Position[Accumulate[DivisorSigma[0,Range[210000]]^2],_?(IntegerQ[ Sqrt[ #]]&)]//Flatten (* _Harvey P. Dale_, Jul 17 2016 *)
%o (PARI) s=0;for(n=1,1e7,if(issquare(s+=numdiv(n)^2),print1(n", "))) \\ _Charles R Greathouse IV_, Feb 21 2011
%Y Cf. A000005.
%K nonn
%O 1,2
%A _Ctibor O. Zizka_, Feb 21 2011