21291581 has 4 divisors (see below), whose sum is σ = 21447132. Its totient is φ = 21136032.

The previous prime is 21291547. The next prime is 21291593. The reversal of 21291581 is 18519212.

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

It can be written as a sum of positive squares in 2 ways, for example, as 6584356 + 14707225 = 2566^2 + 3835^2 .

It is a cyclic number.

It is not a de Polignac number, because 21291581 - 2¹⁴ = 21275197 is a prime.

It is a super-2 number, since 2×21291581² = 906662842959122, 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 (21291511) 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, 77570 + ... + 77843.

It is an arithmetic number, because the mean of its divisors is an integer number (5361783).

Almost surely, 2^21291581 is an apocalyptic number.

It is an amenable number.

21291581 is a deficient number, since it is larger than the sum of its proper divisors (155551).

21291581 is a wasteful number, since it uses less digits than its factorization.

21291581 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 155550.

The product of its digits is 1440, while the sum is 29.

The square root of 21291581 is about 4614.2801172014. The cubic root of 21291581 is about 277.1634575049.

The spelling of 21291581 in words is "twenty-one million, two hundred ninety-one thousand, five hundred eighty-one".