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 A089653 #26 Sep 05 2023 18:25:51
%S A089653 2,4,5,8,10,16,17,20,25,26,32,37,40,50,52,64,65,80,82,100,101,104,122,
%T A089653 125,128,145,160,164,170,197,200,208,226,244,250,256,257,289,290,320,
%U A089653 325,328,338,340,362,400,401,416,442,452,485,488,500,512,530,577,580
%N A089653 Numbers k > 1 such that (product of prime factors of k) - 1 is a perfect square.
%H A089653 Amiram Eldar, <a href="/A089653/b089653.txt">Table of n, a(n) for n = 1..10000</a>
%e A089653 The prime factors of 10 are 2 and 5 and 2 * 5 - 1 = 9 is a square; so 10 belongs to the sequence.
%t A089653 ppf[n_] := Apply[Times, Transpose[FactorInteger[n]][[1]]]; Select[Range[2, 10^3], IntegerQ[Sqrt[ppf[ # ] - 1]] &]
%o A089653 (PARI) isok(k) = (k>1) && issquare(vecprod(factor(k)[, 1])-1); \\ _Michel Marcus_, Sep 05 2023
%Y A089653 Cf. A007947, A089632. A091419 gives squares, A091420 gives square roots. Subsets: A002496, A003592 \ {1}.
%K A089653 nonn
%O A089653 1,1
%A A089653 _Joseph L. Pe_, Jan 04 2004
%E A089653 Name clarified by _Michel Marcus_, Sep 05 2023