21505457 has 4 divisors (see below), whose sum is σ = 21629940. Its totient is φ = 21380976.

The previous prime is 21505447. The next prime is 21505469. The reversal of 21505457 is 75450512.

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 12117361 + 9388096 = 3481^2 + 3064^2 .

It is a cyclic number.

It is not a de Polignac number, because 21505457 - 2¹⁰ = 21504433 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21505417) 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, 61982 + ... + 62327.

It is an arithmetic number, because the mean of its divisors is an integer number (5407485).

Almost surely, 2^21505457 is an apocalyptic number.

It is an amenable number.

21505457 is a deficient number, since it is larger than the sum of its proper divisors (124483).

21505457 is a wasteful number, since it uses less digits than its factorization.

21505457 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 124482.

The product of its (nonzero) digits is 7000, while the sum is 29.

The square root of 21505457 is about 4637.3976538572. The cubic root of 21505457 is about 278.0884119188.

Adding to 21505457 its reverse (75450512), we get a palindrome (96955969).

The spelling of 21505457 in words is "twenty-one million, five hundred five thousand, four hundred fifty-seven".