What do bibliotherapy, bituminous materials, and the history of Le Flore County, OK, have in common? Give up? Their Dewey numbers (615.8516, 691.96, and 976.679, respectively) are palindromes: they read the same frontwards and backwards. (That decimal point really is just an aid for chunking the number and does not count as an integral part of it.)
How long is the longest Dewey number that is a palindrome? Having tested all the Dewey numbers in the DDC 23 EN database (the database associated with the English-language version of DDC 23) and hundreds upon hundreds of thousands of DDC 23 numbers assigned to materials in WorldCat, I was stunned that the longest palindromic DDC numbers I found were only 7 digits long. In this regard, 615.8516 is joined by 331.4133, 342.0243, 363.7363, and 621.3126.
Many valid Dewey numbers have never been used or even built. Surely it is possible to build Dewey numbers that are palindromes that are longer than those found so far.
I feel a contest coming on. Who can build the longest valid palindromic Dewey number? Send me your candidates by January 31 (at email@example.com), and we’ll declare the winner early in February.