cp's OEIS Frontend

A308657 Smallest number that is nontrivially palindromic in n consecutive number bases.

%I A308657 #20 Aug 26 2019 08:44:43
%S A308657 3,10,178
%N A308657 Smallest number that is nontrivially palindromic in n consecutive number bases.
%C A308657 Nontrivially palindromic means having at least 2 digits in the palindromic base representation.
%C A308657 | n | term | consecutive palindromic bases representations |
%C A308657 +---+------+-----------------------------------------------+
%C A308657 | 1 |   3  | 11_2                                          |
%C A308657 | 2 |  10  | 101_3 = 22_4                                  |
%C A308657 | 3 | 178  | 454_6 = 343_7 = 262_8                         |
%C A308657 It is not known if the fourth term exists. The problem can be looked at in context of Diophantine equations, which seem hard.
%H A308657 Mathoverflow, <a href="https://mathoverflow.net/questions/268590/can-a-number-be-palindromic-in-more-than-3-consecutive-number-bases">Can a number be palindromic in more than 3 consecutive number bases?</a>
%e A308657 a(1) = 3 because it is the smallest nontrivial palindrome in some number base: 11 when written in binary.
%e A308657 a(2) = A279092(1) = 10 because it is the smallest nontrivial palindrome in two consecutive number bases, namely, bases 3 and 4: 101 and 22 when written in those number bases, respectively.
%e A308657 a(3) = A279093(1) = 178 since it can be written as a palindrome, in three consecutive number bases, and it is the smallest such number. Those bases are 6, 7, 8 and those representations are 454, 343, 262.
%t A308657 aQ[n_, m_] := SequenceCount[Length[(d = IntegerDigits[n, #])] > 1 && PalindromeQ[d] & /@ Range[2, Ceiling[Sqrt[n]]], Table[True, {m}]] > 0; a[m_] := Module[{n = 2}, While[!aQ[n, m], n++]; n]; Array[a, 3] (* _Amiram Eldar_, Jul 19 2019 *)
%Y A308657 Cf. A002113 (palindromes in base 10).
%Y A308657 Cf. A279092, A279093 (numbers that are nontrivially palindromic in k or more consecutive integer bases with k=2,3; for k>=4, no examples are known).
%K A308657 nonn,bref,hard,base
%O A308657 1,1
%A A308657 _Matej Veselovac_, Jun 14 2019