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%I #6 Jul 31 2020 10:03:56
%S 36,56,63,65,72,84,104,105,126,130,140,156,168,180,210
%N Numbers of the form k = x*y where phi(k) = 3*(phi(x) + phi(y)).
%C There are 35 solutions (x, y) to phi(x*y) = 3*(phi(x) + phi(y)): (4, 14), (4, 18), (5, 13), (5, 21), (5, 26), (5, 28), (5, 36), (5, 42), (6, 6), (6, 14), (7, 9), (7, 18), (8, 13), (8, 21), (9, 7), (9, 14), (10, 13), (10, 21), (12, 13), (13, 5), (13, 8), (13, 10), (13, 12), (14, 4), (14, 6), (14, 9), (18, 4), (18, 7), (21, 5), (21, 8), (21, 10), (26, 5), (28, 5), (36, 5), (42, 5).
%H GUO Rui, ZHAO Xiqing, ZHANG Lixia and XU Hongxin, <a href="http://www.cqvip.com/QK/91365X/201602/668665530.html">The positive integer solutions of euler function phi(mn) = 3^k*(phi(m) + phi(n))</a>
%o (PARI) is(k) = fordiv(k, d, if(eulerphi(k) == 3*(eulerphi(d) + eulerphi(k/d)), return(1))); 0;
%Y Cf. A000010, A336385.
%K nonn,fini,full
%O 1,1
%A _Jinyuan Wang_, Jul 31 2020