21287557 has 4 divisors (see below), whose sum is σ = 21299980. Its totient is φ = 21275136.

The previous prime is 21287551. The next prime is 21287633. The reversal of 21287557 is 75578212.

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 5239521 + 16048036 = 2289^2 + 4006^2 .

It is a cyclic number.

It is not a de Polignac number, because 21287557 - 2¹⁵ = 21254789 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21287551) 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, 3132 + ... + 7237.

It is an arithmetic number, because the mean of its divisors is an integer number (5324995).

Almost surely, 2^21287557 is an apocalyptic number.

It is an amenable number.

21287557 is a deficient number, since it is larger than the sum of its proper divisors (12423).

21287557 is a wasteful number, since it uses less digits than its factorization.

21287557 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 12422.

The product of its digits is 39200, while the sum is 37.

The square root of 21287557 is about 4613.8440589166. The cubic root of 21287557 is about 277.1459955791.

The spelling of 21287557 in words is "twenty-one million, two hundred eighty-seven thousand, five hundred fifty-seven".