21521781 has 24 divisors (see below), whose sum is σ = 33289984. Its totient is φ = 13880160.

The previous prime is 21521771. The next prime is 21521803. The reversal of 21521781 is 18712512.

It is not a de Polignac number, because 21521781 - 2⁷ = 21521653 is a prime.

It is a super-2 number, since 2×21521781² = 926374114823922, which contains 22 as substring.

It is a Harshad number since it is a multiple of its sum of digits (27).

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21521701) by changing a digit.

It is a polite number, since it can be written in 23 ways as a sum of consecutive naturals, for example, 6105 + ... + 8961.

Almost surely, 2^21521781 is an apocalyptic number.

It is an amenable number.

21521781 is a deficient number, since it is larger than the sum of its proper divisors (11768203).

21521781 is an equidigital number, since it uses as much as digits as its factorization.

21521781 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 2903 (or 2891 counting only the distinct ones).

The product of its digits is 1120, while the sum is 27.

The square root of 21521781 is about 4639.1573588315. The cubic root of 21521781 is about 278.1587563455.

The spelling of 21521781 in words is "twenty-one million, five hundred twenty-one thousand, seven hundred eighty-one".