Lychrel number
Unsolved problem in mathematics: Do any base10 Lychrel numbers exist? (more unsolved problems in mathematics)

A Lychrel number is a natural number that cannot form a palindrome through the iterative process of repeatedly reversing its digits and adding the resulting numbers. This process is sometimes called the 196algorithm, after the most famous number associated with the process. In base ten, no Lychrel numbers have been yet proved to exist, but many, including 196, are suspected on heuristic^{[1]} and statistical grounds. The name "Lychrel" was coined by Wade Van Landingham as a rough anagram of Cheryl, his girlfriend's first name.
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Reverseandadd process[edit]
The reverseandadd process produces the sum of a number and the number formed by reversing the order of its digits. For example, 56 + 65 = 121. As another example, 125 + 521 = 646.
Some numbers become palindromes quickly after repeated reversal and addition, and are therefore not Lychrel numbers. All onedigit and twodigit numbers eventually become palindromes after repeated reversal and addition.
About 80% of all numbers under 10,000 resolve into a palindrome in four or fewer steps; about 90% of those resolve in seven steps or fewer. Here are a few examples of nonLychrel numbers:
 56 becomes palindromic after one iteration: 56+65 = 121.
 57 becomes palindromic after two iterations: 57+75 = 132, 132+231 = 363.
 59 becomes a palindrome after 3 iterations: 59+95 = 154, 154+451 = 605, 605+506 = 1111
 89 takes an unusually large 24 iterations (the most of any number under 10,000 that is known to resolve into a palindrome) to reach the palindrome 8,813,200,023,188.
 10,911 reaches the palindrome 4668731596684224866951378664 (28 digits) after 55 steps.
 1,186,060,307,891,929,990 takes 261 iterations to reach the 119digit palindrome 44562665878976437622437848976653870388884783662598425855963436955852489526638748888307835667984873422673467987856626544, which is the current world record for the Most Delayed Palindromic Number. It was solved by Jason Doucette's algorithm and program (using Benjamin Despres' reversaladdition code) on November 30, 2005.
 On January 23, 2017 a Russian schoolboy, Andrey S. Shchebetov, announced on his web site that he had found a sequence of the first 126 numbers (125 of them never reported before) that take exactly 261 steps to reach a 119digit palindrome. This sequence was published in OEIS as A281506. This sequence started with 1,186,060,307,891,929,990  by then the only publicly known number found by Jason Doucette back in 2005. On May 12, 2017 this sequence was extended to 108864 terms in total and included the first 108864 delayed palindromes with 261step delay. The extended sequence ended with 1,999,291,987,030,606,810  its largest and its final term.
 Any number from A281506 could be used as a primary base to construct higher order 261step palindromes. For example, based on 1,999,291,987,030,606,810 the following number 199929198703060681000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000001999291987030606810 also becomes a 238digit palindrome 44562665878976437622437848976653870388884783662598425855963436955852489526638748888307835667984873422673467987856626544 44562665878976437622437848976653870388884783662598425855963436955852489526638748888307835667984873422673467987856626544 after 261 steps.
The smallest known number that is not known to form a palindrome is 196. It is the smallest Lychrel number candidate.
The number resulting from the reversal of the digits of a Lychrel number is also a Lychrel number.
Proof not found[edit]
In other bases (these bases are power of 2, like binary and hexadecimal), certain numbers can be proven to never form a palindrome after repeated reversal and addition,^{[2]} but no such proof has been found for 196 and other base 10 numbers.
It is conjectured that 196 and other numbers that have not yet yielded a palindrome are Lychrel numbers, but no number in base ten has yet been proven to be Lychrel. Numbers which have not been demonstrated to be nonLychrel are informally called "candidate Lychrel" numbers. The first few candidate Lychrel numbers (sequence A023108 in the OEIS) are:
 196, 295, 394, 493, 592, 689, 691, 788, 790, 879, 887, 978, 986, 1495, 1497, 1585, 1587, 1675, 1677, 1765, 1767, 1855, 1857, 1945, 1947, 1997.
The numbers in bold are suspected Lychrel seed numbers (see below). Computer programs by Jason Doucette, Ian Peters and Benjamin Despres have found other Lychrel candidates. Indeed, Benjamin Despres' program has identified all suspected Lychrel seed numbers of less than 17 digits.^{[3]} Wade Van Landingham's site lists the total number of found suspected Lychrel seed numbers for each digit length.^{[4]}
The bruteforce method originally deployed by John Walker has been refined to take advantage of iteration behaviours. For example, Vaughn Suite devised a program that only saves the first and last few digits of each iteration, enabling testing of the digit patterns in millions of iterations to be performed without having to save each entire iteration to a file.^{[5]} However, so far no algorithm has been developed to circumvent the reversal and addition iterative process.
Threads, seed and kin numbers[edit]
The term thread, coined by Jason Doucette, refers to the sequence of numbers that may or may not lead to a palindrome through the reverse and add process. Any given seed and its associated kin numbers will converge on the same thread. The thread does not include the original seed or kin number, but only the numbers that are common to both, after they converge.
Seed numbers are a subset of Lychrel numbers, that is the smallest number of each non palindrome producing thread. A seed number may be a palindrome itself. The first three examples are shown in bold in the list above.
Kin numbers are a subset of Lychrel numbers, that include all numbers of a thread, except the seed, or any number that will converge on a given thread after a single iteration. This term was introduced by Koji Yamashita in 1997.
196 palindrome quest[edit]
Because 196 (base10) is the lowest candidate Lychrel number, it has received the most attention.
In the 1980s the 196 palindrome problem attracted the attention of microcomputer hobbyists, with search programs by Jim Butterfield and others appearing in several massmarket computing magazines.^{[6]}^{[7]}^{[8]} In 1985 a program by James Killman ran unsuccessfully for over 28 days, cycling through 12,954 passes and reaching a 5366digit number.^{[8]}
John Walker began his 196 Palindrome Quest on 12 August 1987 on a Sun 3/260 workstation. He wrote a C program to perform the reversal and addition iterations and to check for a palindrome after each step. The program ran in the background with a low priority and produced a checkpoint to a file every two hours and when the system was shut down, recording the number reached so far and the number of iterations. It restarted itself automatically from the last checkpoint after every shutdown. It ran for almost three years, then terminated (as instructed) on 24 May 1990 with the message:
 Stop point reached on pass 2,415,836.
 Number contains 1,000,000 digits.
196 had grown to a number of one million digits after 2,415,836 iterations without reaching a palindrome. Walker published his findings on the Internet along with the last checkpoint, inviting others to resume the quest using the number reached so far.
In 1995, Tim Irvin and Larry Simkins used a multiprocessor computer and reached the two million digit mark in only three months without finding a palindrome. Jason Doucette then followed suit and reached 12.5 million digits in May 2000. Wade VanLandingham used Jason Doucette's program to reach 13 million digits, a record published in Yes Mag: Canada's Science Magazine for Kids. Since June 2000, Wade VanLandingham has been carrying the flag using programs written by various enthusiasts. By 1 May 2006, VanLandingham had reached the 300 million digit mark (at a rate of one million digits every 5 to 7 days). Using distributed processing,^{[9]} in 2011 Romain Dolbeau completed a billion iterations to produce a number with 413,930,770 digits, and in February 2015 his calculations reached a number with billion digits.^{[10]} A palindrome has yet to be found.
Other potential Lychrel numbers which have also been subjected to the same brute force method of repeated reversal addition include 879, 1997 and 7059: they have been taken to several million iterations with no palindrome being found.^{[11]}
Other bases[edit]
In base 2, 10110 (22 in decimal) has been proven to be a Lychrel number, since after 4 steps it reaches 10110100, after 8 steps it reaches 1011101000, after 12 steps it reaches 101111010000, and in general after 4n steps it reaches a number consisting of 10, followed by n+1 ones, followed by 01, followed by n+1 zeros. This number obviously cannot be a palindrome, and none of the other numbers in the sequence are palindromes.
Lychrel numbers have been proven to exist in the following bases: 11, 17, 20, 26 and all powers of 2.^{[12]}^{[13]}^{[14]}
The smallest number in each base which could possibly be a Lychrel number are (sequence A060382 in the OEIS):
b  Smallest possible Lychrel number in base b written in base b (base 10) 

2  10110 (22) 
3  10201 (100) 
4  3333 (255) 
5  10313 (708) 
6  4555 (1079) 
7  10513 (2656) 
8  1775 (1021) 
9  728 (593) 
10  196 (196) 
11  83A (1011) 
12  179 (237) 
13  CCC (2196) 
14  1BB (361) 
15  1EC (447) 
16  19D (413) 
17  B6G (3297) 
18  1AF (519) 
19  HI (341) 
20  IJ (379) 
21  1CI (711) 
22  KL (461) 
23  LM (505) 
24  MN (551) 
25  1FM (1022) 
26  OP (649) 
27  PQ (701) 
28  QR (755) 
29  RS (811) 
30  ST (869) 
References[edit]
 ^ O'Bryant, Kevin (26 December 2012). "Reply to Status of the 196 conjecture?". Math Overflow.
 ^ Brown, Kevin. "Digit Reversal Sums Leading to Palindromes". MathPages.
 ^ VanLandingham, Wade. "Lychrel Records". p196.org.
 ^ VanLandingham, Wade. "Identified Seeds". p196.org.
 ^ "On NonBrute Force Methods". Archived from the original on 20061015.
 ^ "Bits and Pieces". The Transactor. Transactor Publishing. 4 (6): 16–23. 1984. Retrieved 26 December 2014.
 ^ Rupert, Dale (October 1984). "Commodares: Programming Challenges". Ahoy!. Ion International (10): 23,97–98.
 ^ ^{a} ^{b} Rupert, Dale (June 1985). "Commodares: Programming Challenges". Ahoy!. Ion International (18): 81–84,114.
 ^ Swierczewski, Lukasz; Dolbeau, Romain (June 23, 2014). The p196_mpi Implementation of the ReverseAndAdd Algorithm for the Palindrome Quest. International Supercomputing Conference. Leipzig, Germany.
 ^ Dolbeau, Romain. "The p196_mpi page". www.dolbeau.name.
 ^ "Lychrel Records". Archived from the original on December 5, 2003. Retrieved September 2, 2016.
 ^ See comment section in A060382
 ^ "Digit Reversal Sums Leading to Palindromes".
 ^ "Letter from David Seal".
External links[edit]
 OEIS sequence A023108 (Positive integers which apparently never result in a palindrome under ...)
 John Walker – Three years of computing
 Tim Irvin – About two months of computing
 Jason Doucette – World records – 196 Palindrome Quest, Most Delayed Palindromic Number
 Benjamin Despres
 196 and Other Lychrel Numbers by Wade VanLandingham
 Weisstein, Eric W. "196Algorithm". MathWorld.
 MathPages – Digit Reversal Sums Leading to Palindromes
 NumberPhile