• 66 can be written using four 4's:

• Sorting the digits of 2^{66} in ascending order we obtain a prime of 19 digits.

•
66 is the minimal such that the integers from 1 to
can be arranged in a circle so that the sum of two adjacent
terms is a palindrome with at least 2 digits:

66 has 8 divisors (see below), whose sum is σ = 144. Its totient is φ = 20.

The previous prime is 61. The next prime is 67.

66 is nontrivially palindromic in base 10.

66 is an esthetic number in base 7 and base 12, because in such bases its adjacent digits differ by 1.

66 is a nontrivial binomial coefficient, being equal to C(12, 2).

It is a sphenic number, since it is the product of 3 distinct primes.

66 is an admirable number.

It is a nude number because it is divisible by every one of its digits.

66 is a nontrivial repdigit in base 10.

It is a plaindrome in base 7, base 10, base 12, base 14 and base 15.

It is a nialpdrome in base 3, base 9, base 10, base 11, base 13 and base 16.

It is a zygodrome in base 10.

It is a pernicious number, because its binary representation contains a prime number (2) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 1 + ... + 11.

It is equal to the Eulerian number *A*(5, 2).

It is an arithmetic number, because the mean of its divisors is an integer number (18).

66 is the 11-th triangular number and also the 6-th hexagonal number.

It is a practical number, because each smaller number is the sum of distinct divisors of 66, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (72).

66 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.

66 is a wasteful number, since it uses less digits than its factorization.

66 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 16.

The product of its digits is 36, while the sum is 12.

The square root of 66 is about 8.1240384046. The cubic root of 66 is about 4.0412400206.

Adding to 66 its sum of digits (12), we get a triangular number (78 = T_{12}).

The spelling of 66 in words is "sixty-six", and thus it is an aban number, an eban number, an oban number, and an uban number.

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