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A274433 Products of two distinct tribonacci numbers > 1. 3
15, 27, 45, 51, 85, 93, 153, 155, 171, 279, 285, 315, 513, 525, 527, 579, 945, 965, 969, 1065, 1737, 1767, 1775, 1785, 1959, 3195, 3255, 3265, 3281, 3603, 5877, 5983, 5985, 6005, 6035, 6627, 10809, 11001, 11005, 11045, 11101, 12189, 19881, 20235, 20243 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Are these unique among all products of distinct tribonacci numbers (A000213)?  (See A274432.)

LINKS

Table of n, a(n) for n=1..45.

EXAMPLE

The tribonacci numbers > 1 are 3,5,9,17,31,57,..., so that the binary products in increasing order are 15, 27,45, 51, 85, ...

MATHEMATICA

r[1] := 1; r[2] := 1; r[3] = 1; r[n_] := r[n] = r[n - 1] + r[n - 2] + r[n - 3];

s = {1}; z = 60; f = Map[r, Range[z]]; Take[f, 20]

Do[s = Union[s, Select[s*f[[i]], # <= f[[z]] &]], {i, z}];

Take[s, 2 z]  (* A274432 *)

infQ[n_] := MemberQ[f, n];

ans = Table[#[[Flatten[Position[Map[Apply[Times, #] &, #], s[[n]]]][[1]]]] &[

Rest[Subsets[Map[#[[1]] &, Select[Map[{#, infQ[#]} &, Divisors[s[[n]]]], #[[2]] && #[[1]] > 1 &]]]]], {n, 2, 300}];

Map[Apply[Times, #] &, Select[ans, Length[#] == 2 &]] (* A274433 *)

Map[Apply[Times, #] &, Select[ans, Length[#] == 3 &]] (* A274434 *)

(* Peter J. C. Moses, Jun 17 2016 *)

CROSSREFS

Cf. A000213, A274432, A274434.

Sequence in context: A097222 A004069 A110978 * A227804 A343139 A087719

Adjacent sequences:  A274430 A274431 A274432 * A274434 A274435 A274436

KEYWORD

nonn,easy

AUTHOR

Clark Kimberling, Jun 22 2016

STATUS

approved

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Last modified July 24 20:26 EDT 2021. Contains 346273 sequences. (Running on oeis4.)