21308781 has 16 divisors (see below), whose sum is σ = 31426304. Its totient is φ = 12757824.

The previous prime is 21308767. The next prime is 21308789. The reversal of 21308781 is 18780312.

It is a happy number.

It is not a de Polignac number, because 21308781 - 2⁷ = 21308653 is a prime.

It is a super-2 number, since 2×21308781² = 908128295411922, which contains 22 as substring.

It is a Curzon number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21308789) by changing a digit.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 5941 + ... + 8826.

It is an arithmetic number, because the mean of its divisors is an integer number (1964144).

Almost surely, 2^21308781 is an apocalyptic number.

It is an amenable number.

21308781 is a deficient number, since it is larger than the sum of its proper divisors (10117523).

21308781 is a wasteful number, since it uses less digits than its factorization.

21308781 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 14820.

The product of its (nonzero) digits is 2688, while the sum is 30.

The square root of 21308781 is about 4616.1435202992. The cubic root of 21308781 is about 277.2380711647.

The spelling of 21308781 in words is "twenty-one million, three hundred eight thousand, seven hundred eighty-one".