21523781 has 4 divisors (see below), whose sum is σ = 21552972. Its totient is φ = 21494592.

The previous prime is 21523769. The next prime is 21523793. The reversal of 21523781 is 18732512.

It is a semiprime because it is the product of two primes.

It is an interprime number because it is at equal distance from previous prime (21523769) and next prime (21523793).

It can be written as a sum of positive squares in 2 ways, for example, as 10857025 + 10666756 = 3295^2 + 3266^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21523781 is a prime.

It is a super-2 number, since 2×21523781² = 926546297071922, which contains 22 as substring.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21523741) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 13460 + ... + 14973.

It is an arithmetic number, because the mean of its divisors is an integer number (5388243).

Almost surely, 2^21523781 is an apocalyptic number.

It is an amenable number.

21523781 is a deficient number, since it is larger than the sum of its proper divisors (29191).

21523781 is an equidigital number, since it uses as much as digits as its factorization.

21523781 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 29190.

The product of its digits is 3360, while the sum is 29.

The square root of 21523781 is about 4639.3729102110. The cubic root of 21523781 is about 278.1673724274.

The spelling of 21523781 in words is "twenty-one million, five hundred twenty-three thousand, seven hundred eighty-one".