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 A143421 #12 Apr 23 2016 09:50:43 %S A143421 1,1,1,2,1,1,2,1,2,2,1,3,1,1,1,3,3,2,1,1,2,1,1,1,1,4,1,1,1,6,1,2,1,2, %T A143421 2,1,4,2,1,1,1,4,1,2,1,6,1,1,1,3,1,1,1,5,1,1,3,2,2,1,1,4,1,2,1,5,1,1, %U A143421 4,1,1,3,1,1,1,1,7,2,1,2,1,1,2,1,10,1,4,1,1,1,3,1,1,2,4,3,1,6,1,1,1,2,1,1,6 %N A143421 Number of odd numbers k such that phi(k) = n, where n runs through the values (A002202) taken by phi. %C A143421 The first zero term is for n = 16842752 = 257*2^16. If there are only five Fermat primes, then terms will be zero for n=2^r for all r>31. This is discussed in problem E3361. %C A143421 a(2698482) = 0. That is, the 2698482nd term of A002202 is 16842752. - _T. D. Noe_, Aug 19 2008 %D A143421 R. K. Guy, Unsolved problems in number theory, B39. %H A143421 T. D. Noe, <a href="/A143421/b143421.txt">Table of n, a(n) for n=1..10000</a> %H A143421 T. D. Noe, <a href="http://www.sspectra.com/math/16842752.txt">Numbers Like 16842752</a> %H A143421 William P. Wardlaw, L. L. Foster and R. J. Simpson, <a href="http://www.jstor.org/stable/2323869">Problem E3361</a>, Amer. Math. Monthly, Vol. 98, No. 5 (May, 1991), 443-444. %F A143421 A058277(n) = A143421(n) + A143422(n). %K A143421 nonn %O A143421 1,4 %A A143421 _T. D. Noe_, Aug 14 2008