A215782 - OEIS

%I #20 Feb 21 2020 20:04:13

%S 27,81,91,103,135,163,189,199,203,221,243,247,273,297,305,309,351,371,

%T 377,397,405,421,455,459,489,513,515,551,559,567,597,609,621,637,663,

%U 675,721,729,741,757,779,783,815,819,837,883,891,915,927,945,991,995,999

%N Odd tribonacci non-divisors.

%C Odd numbers that do not divide any term of A000213. - _N. J. A. Sloane_, Aug 23 2012

%C There are several sequences that have the name "tribonacci". Sequence A000073 is another, but that one has terms divisible by any number. - _T. D. Noe_, Aug 28 2012

%H T. D. Noe, <a href="/A215782/b215782.txt">Table of n, a(n) for n = 1..1000</a>

%H Project Euler, <a href="http://projecteuler.net/problem=225">Problem #225: Tribonacci non-divisors</a>

%t t = {}; Do[a = {1, 1, 1}; a0 = a; done = False; While[s = Total[a]; a = RotateLeft[a]; a[[-1]] = Mod[s, n]; If[a[[-1]] == 0, done = True]; a != a0 && ! done]; If[! done, AppendTo[t, n]], {n, 1, 1087, 2}]; t (* _T. D. Noe_, Aug 28 2012 *)

%o (PARI) is(n)=if(n%2==0 || n<9, return(0)); my(a=1,b=1,c=3,t=4); while(c!=1 || b!=1 || a!=1, [a,b,c]=[b,c,(a+b+c)%n]; if(c==0, return(0))); 1 \\ _Charles R Greathouse IV_, Jan 12 2017

%Y Cf. A000213, A046735.

%K nonn

%O 1,1

%A _V. Raman_, Aug 23 2012