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 A280587 #71 Apr 03 2021 08:43:05 %S A280587 81,121,343,361,529,649,841,961,1219,1331,1537,1633,1837,1849,1909, %T A280587 1969,2047,2209,2401,2497,2773,2809,2959,3113,3127,3151,3223,3421, %U A280587 3481,3487,3841,3901,3953,4189,4321,4399,4531,4661,4741,4829,4897,4913,5041,5129,5137,5191,5269,5401,5539,5753 %N A280587 Composite numbers k such that phi(x) = phi(k) has only 2 solutions: x = k and x = 2*k. %C A280587 All terms are odd. %C A280587 If q > 7 is in A005385, then q^2 is in the sequence. %C A280587 The sequence has a positive natural density. - Information from Carl Pomerance, Jan 07 2017 %H A280587 Amiram Eldar, <a href="/A280587/b280587.txt">Table of n, a(n) for n = 1..10000</a> %Y A280587 Cf. A000010, A005385, A058340 (such primes). %K A280587 nonn %O A280587 1,1 %A A280587 _Thomas Ordowski_ and _Altug Alkan_, Jan 06 2017