21578 has 4 divisors (see below), whose sum is σ = 32370. Its totient is φ = 10788.

The previous prime is 21577. The next prime is 21587. The reversal of 21578 is 87512.

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

It can be written as a sum of positive squares in only one way, i.e., 18769 + 2809 = 137^2 + 53^2 .

It is a super-2 number, since 2×21578² = 931220168, which contains 22 as substring.

It is not an unprimeable number, because it can be changed into a prime (21577) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 5393 + ... + 5396.

2²¹⁵⁷⁸ is an apocalyptic number.

21578 is a deficient number, since it is larger than the sum of its proper divisors (10792).

21578 is a wasteful number, since it uses less digits than its factorization.

21578 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 10791.

The product of its digits is 560, while the sum is 23.

The square root of 21578 is about 146.8945199795. The cubic root of 21578 is about 27.8400747014.

The spelling of 21578 in words is "twenty-one thousand, five hundred seventy-eight".