21193681 has 4 divisors (see below), whose sum is σ = 21203812. Its totient is φ = 21183552.

The previous prime is 21193651. The next prime is 21193691. The reversal of 21193681 is 18639112.

It is a semiprime because it is the product of two primes, and also a brilliant number, because the two primes have the same length.

It can be written as a sum of positive squares in 2 ways, for example, as 18662400 + 2531281 = 4320^2 + 1591^2 .

It is a cyclic number.

It is not a de Polignac number, because 21193681 - 2¹¹ = 21191633 is a prime.

It is a super-2 number, since 2×21193681² = 898344228659522, which contains 22 as substring.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21193621) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 636 + ... + 6541.

It is an arithmetic number, because the mean of its divisors is an integer number (5300953).

Almost surely, 2^21193681 is an apocalyptic number.

It is an amenable number.

21193681 is a deficient number, since it is larger than the sum of its proper divisors (10131).

21193681 is an equidigital number, since it uses as much as digits as its factorization.

21193681 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 10130.

The product of its digits is 2592, while the sum is 31.

The square root of 21193681 is about 4603.6595225972. The cubic root of 21193681 is about 276.7379998643.

The spelling of 21193681 in words is "twenty-one million, one hundred ninety-three thousand, six hundred eighty-one".