21487781 has 4 divisors (see below), whose sum is σ = 24557472. Its totient is φ = 18418092.

The previous prime is 21487751. The next prime is 21487783. The reversal of 21487781 is 18778412.

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 not a de Polignac number, because 21487781 - 2⁶ = 21487717 is a prime.

It is a super-2 number, since 2×21487781² = 923449464607922, 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 (21487783) 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, 1534835 + ... + 1534848.

It is an arithmetic number, because the mean of its divisors is an integer number (6139368).

Almost surely, 2^21487781 is an apocalyptic number.

It is an amenable number.

21487781 is a deficient number, since it is larger than the sum of its proper divisors (3069691).

21487781 is an equidigital number, since it uses as much as digits as its factorization.

21487781 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 3069690.

The product of its digits is 25088, while the sum is 38.

The square root of 21487781 is about 4635.4914518312. The cubic root of 21487781 is about 278.0122012139.

The spelling of 21487781 in words is "twenty-one million, four hundred eighty-seven thousand, seven hundred eighty-one".