21087781 has 32 divisors (see below), whose sum is σ = 26320896. Its totient is φ = 16632000.

The previous prime is 21087779. The next prime is 21087809. The reversal of 21087781 is 18778012.

It is not a de Polignac number, because 21087781 - 2¹ = 21087779 is a prime.

It is a super-2 number, since 2×21087781² = 889389015007922, 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 (21087751) by changing a digit.

It is a polite number, since it can be written in 31 ways as a sum of consecutive naturals, for example, 296976 + ... + 297046.

It is an arithmetic number, because the mean of its divisors is an integer number (822528).

Almost surely, 2^21087781 is an apocalyptic number.

It is an amenable number.

21087781 is a deficient number, since it is larger than the sum of its proper divisors (5233115).

21087781 is a wasteful number, since it uses less digits than its factorization.

21087781 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 193.

The product of its (nonzero) digits is 6272, while the sum is 34.

The square root of 21087781 is about 4592.1433993289. The cubic root of 21087781 is about 276.2762977360.

It can be divided in two parts, 2108 and 7781, that added together give a palindrome (9889).

The spelling of 21087781 in words is "twenty-one million, eighty-seven thousand, seven hundred eighty-one".