21751381 has 4 divisors (see below), whose sum is σ = 23030892. Its totient is φ = 20471872.

The previous prime is 21751369. The next prime is 21751391. The reversal of 21751381 is 18315712.

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 15476356 + 6275025 = 3934^2 + 2505^2 .

It is a cyclic number.

It is not a de Polignac number, because 21751381 - 2²³ = 13362773 is a prime.

It is a super-2 number, since 2×21751381² = 946245150814322, 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 (21751351) 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, 639730 + ... + 639763.

It is an arithmetic number, because the mean of its divisors is an integer number (5757723).

Almost surely, 2^21751381 is an apocalyptic number.

It is an amenable number.

21751381 is a deficient number, since it is larger than the sum of its proper divisors (1279511).

21751381 is a wasteful number, since it uses less digits than its factorization.

21751381 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1279510.

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

The square root of 21751381 is about 4663.8375829353. The cubic root of 21751381 is about 279.1444163549.

The spelling of 21751381 in words is "twenty-one million, seven hundred fifty-one thousand, three hundred eighty-one".