The OEIS is supported by the many generous donors to the OEIS Foundation.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A371805 Composite numbers k that divide A001644(k) - 1. 0
 182, 25201, 54289, 63618, 194390, 750890, 804055, 1889041, 2487941, 3542533, 3761251, 6829689, 12032021, 12649337, 18002881 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS If k is prime, k divides A001644(k) - 1; and since A001644(k) satisfies a tribonacci recurrence, these numbers could be called tribonacci pseudoprimes. LINKS Table of n, a(n) for n=1..15. EXAMPLE (A001644(182)-1)/182 = 8056145960961609628091266244940745410646318417. MAPLE A001644:=proc(n) option remember: if n=0 then 3 elif n=1 then 1 elif n=2 then 3 else A001644(n-1)+A001644(n-2)+A001644(n-3) fi end: test:=n->A001644(n) mod n = 1:select(test and not isprime, [seq(n, n=1..100000)]); MATHEMATICA seq[kmax_] := Module[{x = 1, y = 3, z = 7, s = {}, t}, Do[t = x + y + z; If[Mod[t, k] == 1 && CompositeQ[k], AppendTo[s, k]]; x = y; y = z; z = t, {k, 4, kmax}]; s]; seq[200000] (* Amiram Eldar, Apr 06 2024 *) PROG (Python) from sympy import isprime from itertools import count, islice def agen(): # generator of terms t0, t1, t2 = 3, 1, 3 for k in count(1): t0, t1, t2 = t1, t2, t0+t1+t2 if k > 1 and not isprime(k) and (t0-1)%k == 0: yield k print(list(islice(agen(), 5))) # Michael S. Branicky, Apr 07 2024 CROSSREFS Cf. A001644. Cf. A005845 (composite k that divide Lucas(k) - 1). Cf. A013998 (composite k that divide Perrin(k) - 1). Sequence in context: A035839 A048546 A225712 * A015306 A190830 A145525 Adjacent sequences: A371802 A371803 A371804 * A371806 A371807 A371808 KEYWORD nonn,more AUTHOR Robert FERREOL, Apr 06 2024 EXTENSIONS a(13)-a(15) from Amiram Eldar, Apr 07 2024 STATUS approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recents
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified August 4 10:16 EDT 2024. Contains 374914 sequences. (Running on oeis4.)