21503681 has 4 divisors (see below), whose sum is σ = 21528000. Its totient is φ = 21479364.

The previous prime is 21503653. The next prime is 21503693. The reversal of 21503681 is 18630512.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4.

It is a cyclic number.

It is not a de Polignac number, because 21503681 - 2⁶ = 21503617 is a prime.

It is a super-2 number, since 2×21503681² = 924816593099522, which contains 22 as substring.

It is a Duffinian number.

It is a self number, because there is not a number n which added to its sum of digits gives 21503681.

It is not an unprimeable number, because it can be changed into a prime (21503621) 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, 10781 + ... + 12618.

It is an arithmetic number, because the mean of its divisors is an integer number (5382000).

Almost surely, 2^21503681 is an apocalyptic number.

It is an amenable number.

21503681 is a deficient number, since it is larger than the sum of its proper divisors (24319).

21503681 is an equidigital number, since it uses as much as digits as its factorization.

21503681 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 24318.

The product of its (nonzero) digits is 1440, while the sum is 26.

The square root of 21503681 is about 4637.2061631978. The cubic root of 21503681 is about 278.0807565190.

The spelling of 21503681 in words is "twenty-one million, five hundred three thousand, six hundred eighty-one".