21155381 has 4 divisors (see below), whose sum is σ = 22782732. Its totient is φ = 19528032.

The previous prime is 21155371. The next prime is 21155399. The reversal of 21155381 is 18355112.

It is a happy number.

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 8439025 + 12716356 = 2905^2 + 3566^2 .

It is a cyclic number.

It is not a de Polignac number, because 21155381 - 2¹⁸ = 20893237 is a prime.

It is a super-2 number, since 2×21155381² = 895100290510322, which contains 22 as substring.

It is a Duffinian number.

It is a self number, because there is not a number n which added to its sum of digits gives 21155381.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21155371) 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, 813656 + ... + 813681.

It is an arithmetic number, because the mean of its divisors is an integer number (5695683).

Almost surely, 2^21155381 is an apocalyptic number.

It is an amenable number.

21155381 is a deficient number, since it is larger than the sum of its proper divisors (1627351).

21155381 is a wasteful number, since it uses less digits than its factorization.

21155381 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1627350.

The product of its digits is 1200, while the sum is 26.

The square root of 21155381 is about 4599.4979073808. The cubic root of 21155381 is about 276.5711976858.

The spelling of 21155381 in words is "twenty-one million, one hundred fifty-five thousand, three hundred eighty-one".