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 A245202 #27 Apr 27 2024 03:35:52 %S A245202 3,9,21,24,26,30,51,72,77,84,90,93,100,119,122,162,168,174,194,210, %T A245202 213,221,276,282,291,301,381,384,386,408,414,437,469,510,527,533,564, %U A245202 594,597,616,723,731,744,770,791,794,858,869,896,917,930,948,952,954 %N A245202 Numbers k such that tau(k) + phi(k) is a perfect square. %C A245202 Numbers k such that A000010(k) + A000005(k) is a perfect square. %H A245202 Amiram Eldar, <a href="/A245202/b245202.txt">Table of n, a(n) for n = 1..10000</a> (terms 1..5000 from Chai Wah Wu) %e A245202 3 is in the sequence because phi(3) + tau(3) = 2 + 2 = 4^2. %e A245202 9 is in the sequence because phi(9) + tau(9) = 6 + 3 = 3^2. %e A245202 15 is not in the sequence because phi(15) + tau(15) = 8 + 4 = 12 = 2^2 * 3, which is not a perfect square. %t A245202 Select[Range[1000], IntegerQ[Sqrt[DivisorSigma[0, #] + EulerPhi[#]]] &] (* _Amiram Eldar_, Apr 27 2024 *) %o A245202 (PARI) isok(n) = issquare(numdiv(n) + eulerphi(n)); \\ _Michel Marcus_, Jul 23 2014 %o A245202 (Python) %o A245202 from sympy import totient, divisor_count %o A245202 from gmpy2 import is_square %o A245202 [n for n in range(1,10**4) if is_square(int(divisor_count(n)+totient(n)))] # _Chai Wah Wu_, Aug 04 2014 %Y A245202 Cf. A000005, A000010. %K A245202 nonn %O A245202 1,1 %A A245202 _Reinhard Muehlfeld_, Jul 13 2014