### Palindrome Numbers

January 31, 2022

Desk Set is an enjoyable 1957

film directed by

Walter Lang and staring

Spencer Tracy and

Katharine Hepburn. The

plot of this film is based on a

fanciful depiction of

mainframe computers and their operation in the late

1950s. This computer, called EMARAC (for Electromagnetic Memory and Research Arithmetical Calculator), could do

natural language processing, and it had a

database that's equivalent to the

Internet of today.

Tracy is portrayed as a

*methods engineer*, but he actually plays something like a

computer scientist who's been assigned to computerize a

library.

Employees resist deployment of the computer, since they

believe that it's intended to replace them, but that didn't happen. The film

acknowledges the assistance of the

International Business Machines Corporation in its

production, and some

critics have called the film an IBM

propaganda piece with the message that computers won't take your job.

For no apparent reason, Tracy gives Hepburn an

IQ test during a

luncheon meeting. Their conversation features two

palindromes. Palindromes are a

word or

phrase that's the same whether

read forwards or backwards. One is

Adam's supposed first words to

Eve,

*Madam, I'm Adam*. The other is something

Napoleon might have said,

*Able was I ere I saw Elba*,

Elba being the place of Napoleon's first

exile from 1814 to 1815.

It's not surprising that there are an

infinite number of

palindromic numbers, and creating such numbers is all too easy. It's more fun finding

prime numbers that are also palindromes. These

palindrome primes are

integer sequence A002385 of the

On-Line Encyclopedia of Integer Sequences. A simple example is the number eleven; a more complex one is 17471. Palindromic primes are found in

number bases other than

base 10 (decimal). The prime number 1991 is also a palindrome when written in

hexadecimal, 7C7.

Just below one million (which is obviously not a palindrome) we have 98689 as a palindrome prime; or, the hexadecimal palindrome prime F383F (997439 in decimal). These are very easily generated in a

high-level language, and my

PHP and C language

programs to calculate palindrome primes can be found

here.

*graph of decimal palindrome primes less than ten million.*

There are 781 decimal palindrome primes up to 9,989,899.

The gaps are a consequence of the fact that every palindrome with an even number of digits is divisible by 11, so these numbers are not prime.

(Data from my C language program, graphed using Gnumeric. Click for larger image.)

After discovering a

mathematical concept,

mathematicians search for possible variants. Such is the case for the

Pythagorean theorem, for which there are more than 370

proofs.[2] My favorite, perhaps because of its simplicity, is shown figure. It's credited to the

12th century Hindu mathematician,

Bhaskara (1114-1185). One proof, thought to have been created by

Leonardo da Vinci (1452-1519), was actually authored in 1772 by

German physicist,

Johannes Tobias Mayer (1752-1830).[3]

*A proof of the Pythagorean theorem by Bhaskara.*

In Bhaskara's proof, the area of the inner square is c^{2}, the area of the outer square is (a+b)^{2}, and the area of each triangle is (1/2)(ab).

U.S. President, James Garfield (1831-1881), created a novel proof when he was a member of the U.S. House of Representatives. His proof equates the area of a trapezoid to the combined area of its contained triangles.

(Drawing by the author using Inkscape. Click for larger image.)

Mathematician,

Daniel Tsai, has examined a variant of palindrome numbers that he calls

*v-palindromes*.[4] The numbers themselves are not palindromes; rather, the palindrome property arises from the fact that these numbers and the numbers formed by reversing their digits have the same sum of their

factors. For example,

198 = 2 · 3 · 2 · 11

2 + 3 + 2 + 11 = 18

891 = 3 · 4 · 11

3 + 4 + 11 = 18

Tsai writes that he first discovered his

*v-palindromes* in 2007, when he was just 15 years old.[4] His first

publication on the topic was in October, 2018, in Sṓgaku Seminar magazine, which he describes as the

American Mathematical Monthly of

Japan.[5] This paper also proved that there are infinitely many

*v-palindromes* in base ten.[4] This is a simple proof in which it is shown that numbers with any number of nines in the middle, are v-palindromes in base ten (e.g., 18, 198, 1998).[4] Furthermore, all numbers with any number of 18's repeatedly

concatenated are v-palindromes in base ten (e.g., 18, 1818, 181818).[4]

__The smallest v-palindrome for number bases to 16__
Number Base |
Smallest v-palindrome (decimal equivalent) |

2 |
175 |

3 |
1280 |

4 |
6 |

5 |
288 |

6 |
10 |

7 |
731 |

8 |
14 |

9 |
93 |

10 |
18 |

11 |
135 |

12 |
22 |

13 |
63 |

14 |
26 |

15 |
291 |

16 |
109 |

I wrote about

base 26 (hexavigesimal) numbers in a

recent article (Shakespearean Monkeys, September 20, 2021). This is an interesting number base, since there are 26

lower case characters in the

English alphabet. It's possible to

encode a

letter as base 26 numbers, and this was the basis of an early

Internet cryptography joke.

ROT13 is a

Caesar cipher in which characters are

transposed by 13 places; thus,

**a** becomes

**n**, etc.. ROT13 has the property that doing this operation again restores the original message. ROT13 is used not as a secure

cipher, but as a simple means of giving/retrieving "

spoiler"

information, such as the answer to a

puzzle, and there are

browser extensions for ROT13.

You can

confuse,

amuse, and

impress your

friends by including the number, 14841, in your messages. The decimal number 14841, a

composite number (3 x 3 x 17 x 97), is "

lol" in base 26. I created a

C language program to search the first ten million numbers for base 26 palindrome primes. You can get the

source code here. A list of the numbers is found

here. The occurrence of these palindrome primes is shown in the following graph. Compare this gap pattern with that of the decimal palindrome primes shown above.

*Graph of hexavigesimal (Base 26) palindrome primes less than ten million.*

There are 1062 hexavigesimal palindrome primes up to 9998633 (lmmml).

(Data from my C language program, graphed using Gnumeric. Click for larger image.)

### References:

- Desk Set (1957) Walter Lang, Director, on the Internet Movie Database.
- Elisha Scott Loomis, "The Pythagorean proposition: its demonstrations analyzed and classified, and bibliography of sources for data of the four kinds of proofs," Education Resources Information Center. Institute of Education Sciences (IES) of the U.S. Department of Education (1940); reprinted in 1968 by National Council of Teachers of Mathematics (310 page PDF File).
- Franz Lemmermeyer, "On Leonardo da Vinci's proof of the Theorem of Pythagoras," arXiv, January 15, 2014.
- Daniel Tsai, "v-palindromes: an analogy to the palindromes," arXiv, November 17, 2021.
- Tsai, D. (2018). Natural numbers satisfying an unusual property. Sṓgaku Seminar, vol. 57, no. 11), pp. 35-36 (in Japanese).