OFFSET
1,1
COMMENTS
Appears to be numbers of form 2^a * 3^b, a >= 0, b > 0. - Lekraj Beedassy, Sep 10 2004
This is true: see link "Cyclotomic trinomials". - Robert Israel, Jul 14 2015
These are the conjugates of semiprimes, where conjugation is A122111; or Heinz numbers of conjugates of length-2 partitions. - Gus Wiseman, Nov 09 2023
A multiplicative semigroup: if m and n are in the sequence, then so is m*n. - Antti Karttunen, Jul 13 2024
REFERENCES
Jean-Marie De Koninck and Armel Mercier, 1001 Problèmes en Théorie Classique Des Nombres, Problem 733, pp. 74 and 310, Ellipses Paris, 2004.
LINKS
Robert Israel, Table of n, a(n) for n = 1..10000
Robert Israel, Cyclotomic trinomials
FORMULA
A206787(a(n)) = 4. - Reinhard Zumkeller, Feb 12 2012
Sum_{n>=1} 1/a(n) = 1. - Amiram Eldar, Nov 10 2020
EXAMPLE
The 54th cyclotomic polynomial is x^18 - x^9 + 1 which is trinomial, so 54 is in the sequence.
From Gus Wiseman, Nov 09 2023: (Start)
The terms and conjugate semiprimes, showing their respective Heinz partitions, begin:
3: (2) 4: (1,1)
6: (2,1) 6: (2,1)
9: (2,2) 9: (2,2)
12: (2,1,1) 10: (3,1)
18: (2,2,1) 15: (3,2)
24: (2,1,1,1) 14: (4,1)
27: (2,2,2) 25: (3,3)
36: (2,2,1,1) 21: (4,2)
48: (2,1,1,1,1) 22: (5,1)
54: (2,2,2,1) 35: (4,3)
72: (2,2,1,1,1) 33: (5,2)
81: (2,2,2,2) 49: (4,4)
96: (2,1,1,1,1,1) 26: (6,1)
(End)
MAPLE
with(numtheory): a := []; for m from 1 to 3000 do if nops([coeffs(cyclotomic(m, x))])=3 then a := [op(a), m] fi od; print(a);
MATHEMATICA
max = 5000; Sort[Flatten[Table[2^a 3^b, {a, 0, Floor[Log[2, max]]}, {b, Floor[Log[3, max/2^a]]}]]] (* Alonso del Arte, May 19 2016 *)
PROG
(PARI) isok(n)=my(vp = Vec(polcyclo(n))); sum(k=1, #vp, vp[k] != 0) == 3; \\ Michel Marcus, Jul 11 2015
(PARI) list(lim)=my(v=List(), N); for(n=1, logint(lim\1, 3), N=3^n; while(N<=lim, listput(v, N); N<<=1)); Set(v) \\ Charles R Greathouse IV, Aug 07 2015
CROSSREFS
Differs at the 18th term from A063996.
KEYWORD
nonn
AUTHOR
Len Smiley, Nov 12 2001
EXTENSIONS
Offset set to 1 and more terms from Michel Marcus, Jul 11 2015
STATUS
approved