• 18 can be written using four 4's:
• There are 18 essentially distinct necklaces that contain 5 beads of 3 colors:
18 is nontrivially palindromic in base 5 and base 8.
18 is an esthetic number in base 16, because in such base its adjacent digits differ by 1.
It is a tau number, because it is divible by the number of its divisors (6).
It is an ABA number since it can be written as A⋅BA, here for A=2, B=3.
18 is an idoneal number.
It is a Lucas number.
It is an Ulam number.
It is an alternating number because its digits alternate between odd and even.
It is a Curzon number.
18 is a nontrivial repdigit in base 5 and base 8.
It is a plaindrome in base 5, base 7, base 8, base 10, base 11, base 12, base 13, base 14, base 15 and base 16.
It is a nialpdrome in base 3, base 5, base 6, base 8 and base 9.
It is a zygodrome in base 5 and base 8.
It is a panconsummate number.
18 is the 3-rd heptagonal number.
It is a practical number, because each smaller number is the sum of distinct divisors of 18
18 is a primitive abundant number, since it is smaller than the sum of its proper divisors, none of which is abundant.
It is a pseudoperfect number, because it is the sum of a subset of its proper divisors.
18 is a wasteful number, since it uses less digits than its factorization.
18 is an evil number, because the sum of its binary digits is even.
The square root of 18 is about 4.2426406871. The cubic root of 18 is about 2.6207413942.
Adding to 18 its reverse (81), we get a palindrome (99).