Palindromic number


A palindromic number is a number that remains the same when its digits are reversed. Like 16461, for example, it is "symmetrical". The term palindromic is derived from palindrome, which refers to a word whose spelling is unchanged when its letters are reversed. The first 30 palindromic numbers are:
Palindromic numbers receive most attention in the realm of recreational mathematics. A typical problem asks for numbers that possess a certain property and are palindromic. For instance:
Buckminster Fuller identified a set of numbers he called Scheherazade numbers, some of which have a palindromic symmetry of digit groups.
It is fairly straightforward to appreciate that in any base there are infinitely many palindromic numbers, since in any base the infinite sequence of numbers written as 101, 1001, 10001, etc. consists of palindromic numbers only.

Formal definition

Although palindromic numbers are most often considered in the decimal system, the concept of palindromicity can be applied to the natural numbers in any numeral system. Consider a number n > 0 in base b ≥ 2, where it is written in standard notation with k+1 digits ai as:
with, as usual, 0 ≤ ai < b for all i and ak ≠ 0. Then n is palindromic if and only if ai = ak−i for all i. Zero is written 0 in any base and is also palindromic by definition.

Decimal palindromic numbers

Decimal palindromic numbers with an even number of digits are divisible by 11.
All numbers in base 10 with one digit are palindromic. The number of palindromic numbers with two digits is 9:
There are 90 palindromic numbers with three digits :
and also 90 palindromic numbers with four digits:
so there are 199 palindromic numbers below 104. Below 105 there are 1099 palindromic numbers and for other exponents of 10n we have: 1999, 10999, 19999, 109999, 199999, 1099999, …. For some types of palindromic numbers these values are listed below in a table. Here 0 is included.

Perfect powers

There are many palindromic perfect powers nk, where n is a natural number and k is 2, 3 or 4.
The first nine terms of the sequence 12, 112, 1112, 11112,... form the palindromes 1, 121, 12321, 1234321,...
The only known non-palindromic number whose cube is a palindrome is 2201, and it is a conjecture the fourth root of all the palindrome fourth powers are a palindrome with 100000...000001.
G. J. Simmons conjectured there are no palindromes of form nk for k > 4.

Other bases

Palindromic numbers can be considered in numeral systems other than decimal. For example, the binary palindromic numbers are:
or in decimal: 0, 1, 3, 5, 7, 9, 15, 17, 21, 27, 31, 33, …. The Fermat primes and the Mersenne primes form a subset of the binary palindromic primes.
All numbers are palindromic in an infinite number of bases. But, it's more interesting to consider bases smaller than the number itself - in which case most numbers are palindromic in more than one base, for example,,
In base 7, because 1017 is twice a perfect square, several of its multiples are palindromic squares:
132=202
262=1111
552=4444
1012=10201
1432=24442

In base 18, some powers of seven are palindromic:
70=1
71=7
73=111
74=777
76=12321
79=1367631

And in base 24 the first eight powers of five are palindromic as well:
50=1
51=5
52=11
53=55
54=121
55=5A5
56=1331
57=5FF5
58=14641
5A=15AA51
5C=16FLF61

Any number n is palindromic in all bases b with b ≥ n + 1, and also in base n−1. A number that is non-palindromic in all bases 2 ≤ b < n − 1 is called a strictly non-palindromic number.
A palindromic number in base b that is made up of palindromic sequences of length l arranged in a palindromic order is palindromic in base bl
The square of 13310 in base 30 is 4D302 = KKK30 = 3R362 = DPD36.
In base 24 there are more palindromic squares due to 52 = 11. And squares of all numbers in the form 16...7 are palindromic. 1672 = 1E5E1, 16672 = 1E3K3E1, 166672 = 1E3H8H3E1.

Lychrel process

Non-palindromic numbers can be paired with palindromic ones via a series of operations. First, the non-palindromic number is reversed and the result is added to the original number. If the result is not a palindromic number, this is repeated until it gives a palindromic number. Such number is called "a delayed palindrome".
It is not known whether all non-palindromic numbers can be paired with palindromic numbers in this way. While no number has been proven to be unpaired, many do not appear to be. For example, 196 does not yield a palindrome even after 700,000,000 iterations. Any number that never becomes palindromic in this way is known as a Lychrel number.
For example, 1,186,060,307,891,929,990 takes to reach the 119-digit palindrome 44562665878976437622437848976653870388884783662598425855963436955852489526638748888307835667984873422673467987856626544, which is the current world record for the . It was solved by Jason Doucette's algorithm and program on November 30, 2005.
On January 24, 2017 the number 1,999,291,987,030,606,810 was published in OEIS as and announced "The Largest Known Most Delayed Palindrome". The sequence of 125 261-step most delayed palindromes preceding 1,999,291,987,030,606,810 and not reported before was published separately as.

Sum of the reciprocals

The sum of the reciprocals of the palindromic numbers is a convergent series, whose value is approximately 3.37028....

Scheherazade numbers

Scheherazade numbers are a set of numbers identified by Buckminster Fuller in his book Synergetics. Fuller does not give a formal definition for this term, but from the examples he gives, it can be understood to be those numbers that contain a factor of the primorial n#, where n≥13 and is the largest prime factor in the number. Fuller called these numbers Scheherazade numbers because they must have a factor of 1001. Scheherazade is the storyteller of One Thousand and One Nights, telling a new story each night to delay her execution. Since n must be at least 13, the primorial must be at least 1·2·3·5·7·11·13, and 7×11×13 = 1001. Fuller also refers to powers of 1001 as Scheherazade numbers. The smallest primorial containing Scheherazade number is 13# = 30,030.
Fuller pointed out that some of these numbers are palindromic by groups of digits. For instance 17# = 510,510 shows a symmetry of groups of three digits. Fuller called such numbers Scheherazade Sublimely Rememberable Comprehensive Dividends, or SSRCD numbers. Fuller notes that 1001 raised to a power not only produces sublimely rememberable numbers that are palindromic in three-digit groups, but also the values of the groups are the binomial coefficients. For instance,
This sequence fails at 13 because there is a carry digit taken into the group to the left in some groups. Fuller suggests writing these spillovers on a separate line. If this is done, using more spillover lines as necessary, the symmetry is preserved indefinitely to any power. Many other Scheherazade numbers show similar symmetries when expressed in this way.

Sums of palindromes

In 2018, a paper was published demonstrating that every positive integer can be written as the sum of three palindromic numbers in every number system with base 5 or greater.