OFFSET
1,1
COMMENTS
From Wolfdieter Lang, Jan 11 2021: (Start)
Conjecture: a(n) == 0 (mod 6) for n >= 1. After division by 6 the sequence becomes [12, 36, 12721, 16921, 19441, 24481, 49681, 61561, 104161, 229321, 255361, 259681, 266401, 291721, 298201, 311041, 331921, ...].
6*A163573 is a subsequence. See A163573 for the proof. Note that not all a(n), for n >= 3, are obtained by 6*A163573. The first such term is a(115) = 31850496, and a(115)/6 = 5308416 which is not a prime number, hence not a term of A163573. (End)
Wells gives a wrong value of a(3): 76236. - Stefano Spezia, Sep 08 2024
REFERENCES
David Wells, The Penguin Dictionary of Curious and Interesting Numbers. Penguin Books, NY, 1986, Revised edition 1987. See p. 129.
LINKS
Jud McCranie, Table of n, a(n) for n = 1..10000
EXAMPLE
phi(72) = phi(78) = phi(84) = phi(90) = 24, so 72 is in the sequence.
PROG
(PARI) isok(k) = #Set(vector(4, i, eulerphi(k+(i-1)*6))) == 1; \\ Michel Marcus, Sep 17 2023
CROSSREFS
KEYWORD
nonn
AUTHOR
Jud McCranie, Dec 27 1999
STATUS
approved