cp's OEIS Frontend

A337803 Odd integers k not divisible by 5, such that dr(k) divides k-1 or k+1, where dr(k) is the additive digital root of k (A010888).

%I A337803 #11 Oct 30 2020 15:50:10
%S A337803 1,11,13,17,19,29,31,37,41,43,47,49,59,67,71,73,83,89,91,97,101,103,
%T A337803 109,119,121,127,131,137,139,143,149,157,161,163,169,173,181,191,193,
%U A337803 199,209,211,217,221,223,227,229,233,239,247,253,263,271,281,283,287,289,299
%N A337803 Odd integers k not divisible by 5, such that dr(k) divides k-1 or k+1, where dr(k) is the additive digital root of k (A010888).
%e A337803 For k = 13, the additive digital root = 4. (12 mod 4) = 0 and (14 mod 4) = 2, and thus 13 is a sequence entry.
%e A337803 For k = 31, the additive digital root = 4. (30 mod 4) = 2 and (32 mod 4) = 0, so 31 is a sequence entry.
%e A337803 For k = 23, the additive digital root = 5. (22 mod 5) = 2 and (24 mod 5) = 4, so 23 is not a sequence entry.
%t A337803 Select[Range[1, 300, 2], !Divisible[#, 5] && (Divisible[# - 1, (dr = Mod[# - 1, 9] + 1)] || Divisible[# + 1, dr]) &] (* _Amiram Eldar_, Oct 02 2020 *)
%o A337803 (PARI) genit(maxx)={if(maxx<11,maxx=11);for (n=1,maxx,if(n%2==0 ||n%5==0,next);dr=(n-1)%9+1;if( (n+1)%dr==0 ||(n-1)%dr==0, print1(n,",")));}
%Y A337803 Cf. A010888 (additive digital roots).
%K A337803 nonn,base
%O A337803 1,2
%A A337803 _Bill McEachen_, Sep 22 2020