21381517 has 4 divisors (see below), whose sum is σ = 22506880. Its totient is φ = 20256156.

The previous prime is 21381491. The next prime is 21381527. The reversal of 21381517 is 71518312.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4.

It is a cyclic number.

It is not a de Polignac number, because 21381517 - 2⁷ = 21381389 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 (21381527) 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, 562653 + ... + 562690.

It is an arithmetic number, because the mean of its divisors is an integer number (5626720).

Almost surely, 2^21381517 is an apocalyptic number.

It is an amenable number.

21381517 is a deficient number, since it is larger than the sum of its proper divisors (1125363).

21381517 is a wasteful number, since it uses less digits than its factorization.

21381517 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1125362.

The product of its digits is 1680, while the sum is 28.

The square root of 21381517 is about 4624.0152465147. The cubic root of 21381517 is about 277.5531570380.

Adding to 21381517 its reverse (71518312), we get a palindrome (92899829).

It can be divided in two parts, 2138 and 1517, that added together give a triangular number (3655 = T₈₅).

The spelling of 21381517 in words is "twenty-one million, three hundred eighty-one thousand, five hundred seventeen".