OFFSET
1,3
LINKS
Paolo Xausa, Table of n, a(n) for n = 1..9094 (rows 1..500 of the triangle, flattened)
L. Alaoglu and P. Erdős, A conjecture in elementary number theory, Bull. Amer. Math. Soc. 50 (1944), 881-882.
EXAMPLE
Triangle begins:
1, 1,
2, 1,
3, 2, 3, 2, 1,
4, 2, 3, 2, 1,
5, 4, 7, 6, 2, 3, 2, 1,
6, 2, 3, 2, 1,
7, 6, 12, 4, 2, 3, 2, 1,
8, 4, 7, 6, 2, 3, 2, 1,
9, 6, 12, 4, 2, 3, 2, 1,
10, 4, 7, 6, 2, 3, 2, 1,
...
For row n=5, for example, we get 5 -> phi(5) = 4 -> sigma(4) = 7 -> phi(7) = 6 -> phi(6) = 2 -> sigma(2) = 3 -> phi(3) = 2 -> phi(2) = 1 (stop).
MATHEMATICA
A334523row[n_]:=Module[{i=0}, NestWhileList[If[Mod[i++, 3]==1, DivisorSigma[1, #], EulerPhi[#]]&, n, i==0||#>1&]]; Array[A334523row, 15] (* Paolo Xausa, Nov 16 2023 *)
CROSSREFS
KEYWORD
nonn,tabf
AUTHOR
N. J. A. Sloane, May 09 2020, following a suggestion from Amiram Eldar.
STATUS
approved