Palindromic numbers and reverse-and-add
A palindromic number reads the same in both directions, like 121 or 12321. Testing one is straightforward: write the digits out, compare the first with the last, the second with the second to last, and so on. If every pair matches, the number is a palindrome. Single-digit numbers have nothing to compare, so 0 through 9 all qualify.
Negative values are conventionally excluded, because reversing −121 would move the sign and the written form no longer matches. This checker reports a negative input as not a palindrome while still telling you whether its absolute value is one, so both readings are covered. Leading zeros do not exist in a number either, so 012 is read as 12 and reported as not a palindrome.
When a number is not already a palindrome, the reverse-and-add process follows. You add the number to its own reverse and, if the result is still not a palindrome, repeat with the new value. 123 needs a single step to reach 444, while 89 takes 24 steps before it lands on 8813200023188. The famous holdout is 196, which has no known palindrome and is the leading Lychrel number candidate. This tool tries up to 30 steps and stops early if a value would pass the safe integer range, where exact arithmetic can no longer be guaranteed.
Frequently Asked Questions
A number such as 121 or 12321 that reads the same forwards and backwards. Every single-digit number is a palindrome, because reversing it gives the same digit.
By convention no, because reversing −121 moves the sign. This checker reports a negative value as not a palindrome and states separately whether its absolute value is one.
You add a number to its own reverse and repeat until the result is a palindrome. 123 gets there in one step as 123+321=444, while 196 has no known palindrome no matter how long you continue.