## Minimal Palindromic Base

### August 5, 2014

We have a simple little problem today: Given an integer *n* > 2, find the minimum *b* > 1 for which *n* base *b* is a palindrome. For instance, *n* = 15 = 1111_{2} = 120_{3} = 33_{4} = 30_{5} = 23_{6} = 21_{7} = 17_{8} = 16_{9} = 15_{10} = 14_{11} = 13_{12} = 12_{13} = 11_{14}; of those, bases 2, 4 and 14 form palindromes, and the least of those is 2, so the correct answer is 2.

Your task is to write a program that calculates the smallest base for which a number *n* is a palindrome. When you are finished, you are welcome to read or run a suggested solution, or to post your own solution or discuss the exercise in the comments below.

Perl routine to do this – takes values in from command line….

Slightly better code (fixed a minor bug!)

The answer will have 2 digits when b > sqrt(n), and will be of the form kk. If we expand kk, we get k*b + k == n, which only has an integer solution when n % k == 0. So, for sqrt(n) > k >= 1, if n%k == 0 then b = n//k – 1 (where // means integer division).

I was wondering if there was an easier way of going from a number in base n to a number in base n+1 that doing a full conversion. A quick look in Knuth showed there was – he describes a method only using subtraction, described by Walter Soden in 1953, for going from octal to decimal that can be generalized.

Incidentally, 0x1000000000000000 is [ 1 6 15 20 15 6 1 ], radix 1023 and 0x1234567890abcdef is [ 578043 565455 578043 ], radix 1506428.

This version expects input in hex – change to “strtoul(argv[1],NULL,0)” to use decimal, octal or hex.

I’ve been having fun with these in Swift — here’s today’s:

Haskell:

Alternative Haskell implementation of the last function:

Another Python implementation. Similar to the Haskell code. No optimizations.

A simpler version of the isPalindrome Haskell function:

A version in Python. I could not find a method, that significantly performs better than brute force. I liked the upbase method from matthew and implemented a version in Python. Using this method is (in Python) slower than brute force.

Racket:

Got some pretty pictures of the minimal palindromic bases over the first 1000 integers and a full write up too: Minimal palindromic base @ jverkamp.com

In ruby …

Been a while good to be back …

Apologies if this appears twice.

August 5th, 2014.c:The solution is known to run on an Apple Power Mac G4 (AGP Graphics) (450MHz processor, 1GB memory) on both Mac OS 9.2.2 (International English) (the solution interpreted using Leonardo IDE 3.4.1) and Mac OS X 10.4.11 (the solution compiled using Xcode 2.2.1).

(I’m just trying to solve the problems posed by this ‘site whilst I try to get a job; I’m well-aware that my solutions are far from the best – but, in my defence, I don’t have any traditional qualifications in computer science :/ )