cp's OEIS Frontend

A069571 Numbers n in which the k-th digit (counted from the right) is nonzero and is either a divisor of k (but not 1 in case k has a single-digit prime divisor) or a multiple of k, for all 1 <= k <= number of digits of n.

%I A069571 #17 Dec 07 2018 17:19:36
%S A069571 1,2,3,4,5,6,7,8,9,21,22,23,24,25,26,27,28,29,41,42,43,44,45,46,47,48,
%T A069571 49,61,62,63,64,65,66,67,68,69,81,82,83,84,85,86,87,88,89,321,322,323,
%U A069571 324,325,326,327,328,329,341,342,343,344,345,346,347,348,349,361,362
%N A069571 Numbers n in which the k-th digit (counted from the right) is nonzero and is either a divisor of k (but not 1 in case k has a single-digit prime divisor) or a multiple of k, for all 1 <= k <= number of digits of n.
%C A069571 This is the subsequence of terms of A069570 with no digit 1 elsewhere than (possibly) in positions k with no prime divisor < 10, i.e., k = 1, 11, 13, 17, 19, ..., 11*11, 127, 131, 137, 139, 11*13, .... - _M. F. Hasler_, Sep 27 2016
%e A069571 54647 is a member but 14647 and 44647 are not members.
%e A069571 13 is a member of A069570 because both 1 and 3 are nonzero, further, 3 is divisible by n = 1 and 1 divides n = 2. 13 is not a member of this sequence because it has a single-digit multiple 3 > 1.
%e A069571 Numbers with "1" in place n = 1 may be in the sequence since 1 divides itself, and 1 does not exceed 1.
%e A069571 21 is a member of this sequence because the digits are nonzero, 1 divides n = 1 and 2 divides n = 2.
%t A069571 Select[Range@ 400, Times @@ Map[Boole, MapIndexed[If[Or[#1 == 0, And[First@ #2 > 1, #1 == 1]], False, Total@ Boole@ {First@ Divisible[#2, #1], First@ Divisible[#1, #2]} > 0] &, Reverse@ IntegerDigits@ #]] > 0 &] (* _Michael De Vlieger_, Sep 28 2016 *)
%Y A069571 Cf. A069570.
%K A069571 nonn,base
%O A069571 1,2
%A A069571 _Amarnath Murthy_, Mar 24 2002
%E A069571 Definition clarified by _M. F. Hasler_, Sep 27 2016
%E A069571 Edited by _N. J. A. Sloane_, Oct 02 2016