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 A204520 #21 Jul 15 2025 14:38:50 %S A204520 0,1,2,3,7,9,18,47,123,161,322,843,2207,2889,5778,15127,39603,51841, %T A204520 103682,271443,710647,930249,1860498,4870847,12752043,16692641, %U A204520 33385282 %N A204520 Numbers such that floor(a(n)^2 / 5) is a square. %C A204520 Also: Numbers whose square, with its last base-5 digit dropped, is again a square. (For the three initial terms whose squares have only one digit in base 5, it is then understood that this yields zero.) %F A204520 a(n) = sqrt(A055812(n)). %F A204520 Empirical g.f.: -x^2*(x+1)*(3*x^6 + 4*x^5 + 14*x^4 - 5*x^3 - 2*x^2 - x-1) / ((x^4 - 4*x^2 - 1)*(x^4 + 4*x^2 - 1)). - _Colin Barker_, Sep 15 2014 %t A204520 Select[Range[0,5*10^6],IntegerQ[Sqrt[Floor[#^2/5]]]&] (* The program generates the first 24 terms of the sequence. *) (* _Harvey P. Dale_, Jul 15 2025 *) %o A204520 (PARI) b=5;for(n=0,2e9,issquare(n^2\b) && print1(n",")) %Y A204520 Cf. A031149, A055812, A204502, A204514, A204516, A204518 and A004275, A001075, A001541 for the analog in bases 10,...,6 and 4, 3, 2. %K A204520 nonn %O A204520 1,3 %A A204520 _M. F. Hasler_, Jan 15 2012