A335121 - OEIS

%I #10 Apr 14 2023 03:47:32

%S 5,7,33,35,55,87,95,175,201,215,219,245,531,747,927,939,1047,1295,

%T 1463,1473,1551,1855,2015,2103,2421,2431,2547,2619,2631,2765,3535,

%U 4833,5067,5215,7655,7743,7851,10503,11127,11307,13055,13707,16247,16593,17805,18471

%N Admirable totient numbers: numbers that are equal to the sum of their iterated phi, with one of them taken with a minus sign.

%C Analogous to A111592 (admirable numbers) as A082897 (perfect totient numbers) is analogous to A000396 (perfect numbers).

%H Amiram Eldar, <a href="/A335121/b335121.txt">Table of n, a(n) for n = 1..171</a> (terms below 10^9)

%e 5 is a term since the values of the iterated phi of 5 are 4, 2 and 1 and 5 = 4 + 2 - 1.

%t admTotQ[n_] := Module[{s = Most @ Rest @ FixedPointList[EulerPhi, n]}, (ab = Plus @@ s - n) > 0 && EvenQ[ab] && ab/2 < n && MemberQ[s, ab/2]]; Select[Range[8000], admTotQ]

%Y Subsequence of A286265.

%Y Cf. A000010, A053478, A082897, A111592, A288452.

%K nonn

%O 1,1

%A _Amiram Eldar_, May 24 2020