21509137 has 12 divisors (see below), whose sum is σ = 23486220. Its totient is φ = 19688448.

The previous prime is 21509119. The next prime is 21509141. The reversal of 21509137 is 73190512.

It can be written as a sum of positive squares in 6 ways, for example, as 8288641 + 13220496 = 2879^2 + 3636^2 .

It is not a de Polignac number, because 21509137 - 2¹⁵ = 21476369 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21509737) by changing a digit.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 22689 + ... + 23617.

It is an arithmetic number, because the mean of its divisors is an integer number (1957185).

Almost surely, 2^21509137 is an apocalyptic number.

It is an amenable number.

21509137 is a deficient number, since it is larger than the sum of its proper divisors (1977083).

21509137 is a wasteful number, since it uses less digits than its factorization.

21509137 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1092 (or 1079 counting only the distinct ones).

The product of its (nonzero) digits is 1890, while the sum is 28.

The square root of 21509137 is about 4637.7944111398. The cubic root of 21509137 is about 278.1042731177.

Adding to 21509137 its reverse (73190512), we get a palindrome (94699649).

The spelling of 21509137 in words is "twenty-one million, five hundred nine thousand, one hundred thirty-seven".