19666 has 4 divisors (see below), whose sum is σ = 29502. Its totient is φ = 9832.

The previous prime is 19661. The next prime is 19681. The reversal of 19666 is 66691.

19666 = T₁₁₃ + T₁₁₄ + T₁₁₅.

It is a happy number.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 66691 = 17 ⋅3923.

It can be written as a sum of positive squares in only one way, i.e., 16641 + 3025 = 129^2 + 55^2 .

It is a plaindrome in base 7.

It is not an unprimeable number, because it can be changed into a prime (19661) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (7) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 4915 + ... + 4918.

2¹⁹⁶⁶⁶ is an apocalyptic number.

19666 is the 115-th centered triangular number.

19666 is a deficient number, since it is larger than the sum of its proper divisors (9836).

19666 is an equidigital number, since it uses as much as digits as its factorization.

19666 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 9835.

The product of its digits is 1944, while the sum is 28.

The square root of 19666 is about 140.2355161862. The cubic root of 19666 is about 26.9922245557.

It can be divided in two parts, 196 and 66, that added together give a palindrome (262).

The spelling of 19666 in words is "nineteen thousand, six hundred sixty-six".