This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A247136 #23 Aug 22 2020 19:42:56 %S A247136 4,9,25,49,119,121,161,169,289,343,361,369,527,529,711,721,833,841, %T A247136 959,961,1081,1127,1241,1369,1681,1695,1767,1849,2047,2209,2809,3281, %U A247136 3335,3481,3553,3713,3721,4207,4489,4633,4681,5041,5047,5215,5329,6241,6713,6887 %N A247136 Numbers for which the root mean square of nontrivial divisors is an integer. %C A247136 All the squares of prime numbers (A001248) have this property but there are other numbers (A247137): 119,161,343,369,527,711,721,833,959,1081... %H A247136 Daniel Lignon and Charles R Greathouse IV, <a href="/A247136/b247136.txt">Table of n, a(n) for n = 1..10000</a> (first 316 terms from Lignon) %F A247136 Trivially a(n) << n^2 log^2 n. - _Charles R Greathouse IV_, Nov 20 2014 %e A247136 119 is a term because the nontrivial divisors of 119 are [7,17] and sqrt((7^2+17^2)/2)= 13 : it's an integer. %t A247136 Select[Range[2,100000],(Not[PrimeQ[#]] && IntegerQ[ RootMeanSquare[ Rest[ Most[ Divisors[#]]]]])&] %o A247136 (PARI) integralRMS(v)=my(t=norml2(v)/#v); denominator(t)==1 && issquare(t) %o A247136 is(n)=my(d=divisors(n)); #d>2 && integralRMS(d[2..#d-1]) \\ _Charles R Greathouse IV_, Nov 20 2014 %Y A247136 Cf. A140480 (numbers for which the root mean square of all divisors is an integer), A247136 (numbers for which the root mean square of proper divisors is an integer) and A023886 (numbers for which the arithmetic mean of nontrivial divisors is an integer). %K A247136 nonn %O A247136 1,1 %A A247136 _Daniel Lignon_, Nov 20 2014