cp's OEIS Frontend

A373407 Smallest positive integer k such that no more than n numbers (formed by multiplying k by a digit) are anagrams of k, or -1 if no such number exists.

%I A373407 #67 Jun 09 2024 09:07:48
%S A373407 1,1035,123876,1402857,1037520684,142857
%N A373407 Smallest positive integer k such that no more than n numbers (formed by multiplying k by a digit) are anagrams of k, or -1 if no such number exists.
%C A373407 For n = 2..6 all terms are divisible by 9.
%C A373407 For n >= 4, a(n) must be divisible by 9, or a(n) = -1, because all anagrams d*k of k for d = 2, 3, 5, 6, 8 and 9 are divisible by 9. Thus there are only 3 values of d, i.e., 1, 4 and 7, for which k*d must not be divisible by 9.
%C A373407 If a(n) exists for n > 1 then 9|a(n). Holds for n = 2 and n = 3 by inspection. Proof for n >= 4: if k*d is an anagram of k where 2 <= d <= 9 then k*d - k = k*(d-1) is a multiple of 9. For this to be true, k must be a multiple of 9 as d is not of the form 1 (mod 3) for all d. - _David A. Corneth_, Jun 04 2024
%C A373407 From _Michael S. Branicky_, Jun 07 2024: (Start)
%C A373407 The following were constructed from multiples of cyclic numbers (cf. A180340, Wikipedia):
%C A373407 a(6)  = 142857 = (10^6 - 1) / 7;
%C A373407 a(7) <= 1304347826086956521739 = 3*(10^22 - 1) / 23;
%C A373407 a(8) <= 1176470588235294 = 2*(10^16 - 1) / 17;
%C A373407 a(9) <= 105263157894736842 = 2*(10^18 - 1) / 19. (End)
%H A373407 Wikipedia, <a href="https://en.wikipedia.org/wiki/Cyclic_number">Cyclic numbers</a>.
%e A373407 a(2) = 1035, because 1035 * 1 = 1035 and 1035 * 3 = 3105 are anagrams of 1035, and no other number 1035 * i with digit i is an anagram of 1035, and no lesser number verifies this property.
%e A373407 Table n, k, set of multipliers.
%e A373407   1   1          [1]
%e A373407   2   1035       [1, 3]
%e A373407   3   123876     [1, 3, 7]
%e A373407   4   1402857    [1, 2, 3, 5]
%e A373407   5   1037520684 [1, 2, 4, 5, 8]
%e A373407   6   142857     [1, 2, 3, 4, 5, 6]
%o A373407 (PARI) isok(k, n) = my(d=vecsort(digits(k))); sum(i=1, 9, vecsort(digits(k*i)) == d) == n; \\ _Michel Marcus_, Jun 04 2024
%Y A373407 Cf. A023086, A023087, A023088, A023089, A023090, A023091, A023092, A023093.
%Y A373407 Cf. A245680, A245682.
%Y A373407 Cf. A090061.
%Y A373407 Cf. A180340, A001913, A036275.
%K A373407 more,nonn,base,fini
%O A373407 1,2
%A A373407 _Jean-Marc Rebert_, Jun 04 2024