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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. 3
1, 2, 36, 144, 576, 3600, 14400, 921600, 1040400, 4161600, 8643600, 34574400, 266342400, 700131600, 2800526400, 179233689600, 202338032400, 809352129600 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

All known terms excluding a(2) are perfect squares.

LINKS

Table of n, a(n) for n=1..18.

EXAMPLE

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.

MAPLE

s:= n -> numtheory:-phi(numtheory:-sigma(n))/numtheory:-sigma(numtheory:-phi(n)):

  a, na, A[1], sA[1]:=1, 1, 1, 1:

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:

CROSSREFS

Cf. A227011, A062401, A062402, A033632, A229238.

Sequence in context: A134785 A143745 A199944 * A007757 A141217 A206688

Adjacent sequences:  A227924 A227925 A227926 * A227928 A227929 A227930

KEYWORD

nonn

AUTHOR

Vladimir Letsko, Oct 09 2013

STATUS

approved

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Last modified May 21 17:33 EDT 2019. Contains 323444 sequences. (Running on oeis4.)