21020781 has 16 divisors (see below), whose sum is σ = 29689344. Its totient is φ = 13201584.

The previous prime is 21020777. The next prime is 21020803. The reversal of 21020781 is 18702012.

It is not a de Polignac number, because 21020781 - 2² = 21020777 is a prime.

It is a super-2 number, since 2×21020781² = 883746467699922, 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 (21020761) 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, 2350 + ... + 6896.

It is an arithmetic number, because the mean of its divisors is an integer number (1855584).

Almost surely, 2^21020781 is an apocalyptic number.

It is an amenable number.

21020781 is a deficient number, since it is larger than the sum of its proper divisors (8668563).

21020781 is a wasteful number, since it uses less digits than its factorization.

21020781 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 4640.

The product of its (nonzero) digits is 224, while the sum is 21.

The square root of 21020781 is about 4584.8425272849. The cubic root of 21020781 is about 275.9833927205.

Adding to 21020781 its reverse (18702012), we get a palindrome (39722793).

The spelling of 21020781 in words is "twenty-one million, twenty thousand, seven hundred eighty-one".