21542221 has 6 divisors (see below), whose sum is σ = 21959210. Its totient is φ = 21133008.

The previous prime is 21542173. The next prime is 21542231. The reversal of 21542221 is 12224512.

It can be written as a sum of positive squares in 3 ways, for example, as 13213225 + 8328996 = 3635^2 + 2886^2 .

It is not a de Polignac number, because 21542221 - 2⁷ = 21542093 is a prime.

It is a super-2 number, since 2×21542221² = 928134571225682, 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 (21542231) by changing a digit.

It is a polite number, since it can be written in 5 ways as a sum of consecutive naturals, for example, 1026 + ... + 6643.

Almost surely, 2^21542221 is an apocalyptic number.

It is an amenable number.

21542221 is a deficient number, since it is larger than the sum of its proper divisors (416989).

21542221 is an frugal number, since it uses more digits than its factorization.

21542221 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 7775 (or 7722 counting only the distinct ones).

The product of its digits is 320, while the sum is 19.

The square root of 21542221 is about 4641.3598222935. The cubic root of 21542221 is about 278.2467875668.

Adding to 21542221 its reverse (12224512), we get a palindrome (33766733).

It can be divided in two parts, 215 and 42221, that added together give a square (42436 = 206²).

The spelling of 21542221 in words is "twenty-one million, five hundred forty-two thousand, two hundred twenty-one".