54

is the integer between.

Math features

 * 54 is an even number.
 * 54 is a composite number and an abundant number.
 * 54 is the sum of three triangular numbers (due to Gauss' proof that every positive integer is the sum of at most 3 triangular numbers): its palindrome, 45, 6, and 3.
 * As 54 is a multiple of 9 and the sum of its digits is 9, 54 is a Harshad number.
 * In octal, 54 is a palindromic number.
 * 54 is a 19-gonal number.
 * It is a generalized taxicab number: equal to Taxicab(2, 3, 3). These are equal to 54: 72 + 22 + 12 = 62 + 32 + 32 = 52 + 52 + 22 = 54.