OFFSET
1,2
COMMENTS
Numbers k such that A124331(k) = k. This is also a subsequence of the records of A124331 (both their values and their positions).
Terms other than 2 are a perfect square. Proof: phi(k) is even for k > 2. So phi(k)+1 is odd for k > 2. d(k) is odd only if k is a perfect square. So for any term k > 2 we need k to be a perfect square. Checking cases <= 2 leaves only 2 as the nonsquare in this sequence. - David A. Corneth, Mar 31 2021
LINKS
David A. Corneth, Table of n, a(n) for n = 1..10000
MATHEMATICA
Select[Join[{1, 2}, Range[2, 420]^2], Divisible[EulerPhi[#] + 1, DivisorSigma[0, #]] &] (* Amiram Eldar, Mar 31 2021 *)
PROG
(PARI) isA342665(n) = !((eulerphi(n)+1) % numdiv(n));
CROSSREFS
KEYWORD
nonn
AUTHOR
Antti Karttunen, Mar 30 2021
STATUS
approved