cp's OEIS Frontend

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.

A227927 Numbers n such that phi(sigma(k))/sigma(phi(k)) < phi(sigma(n))/sigma(phi(n)) for all k < n and n is the smallest positive integer with this property.

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%I A227927 #13 Oct 11 2013 04:10:32
%S A227927 1,2,36,144,576,3600,14400,921600,1040400,4161600,8643600,34574400,
%T A227927 266342400,700131600,2800526400,179233689600,202338032400,809352129600
%N A227927 Numbers n such that phi(sigma(k))/sigma(phi(k)) <  phi(sigma(n))/sigma(phi(n)) for all k < n and n is the smallest positive integer with this property.
%C A227927 All known terms excluding a(2) are perfect squares.
%e A227927 36 is in the sequence because phi(sigma(36))/sigma(phi(36)) = 18/7 and for all k < 36 phi(sigma(k))/sigma(phi(k)) < 18/7.
%p A227927 s:= n -> numtheory:-phi(numtheory:-sigma(n))/numtheory:-sigma(numtheory:-phi(n)):
%p A227927   a,na,A[1],sA[1]:=1,1,1,1:
%p A227927 1;for i from 2 do ss:=s(i): if ss>a then na:=na+1:A[na]:=ss:a:=ss:sA[na]:=i:print(sA[na]) fi od:
%Y A227927 Cf. A227011, A062401, A062402, A033632, A229238.
%K A227927 nonn
%O A227927 1,2
%A A227927 _Vladimir Letsko_, Oct 09 2013