21155531 has 8 divisors (see below), whose sum is σ = 23579280. Its totient is φ = 18862848.

The previous prime is 21155513. The next prime is 21155543. The reversal of 21155531 is 13555112.

It is a happy number.

It is a sphenic number, since it is the product of 3 distinct primes.

It is a cyclic number.

It is not a de Polignac number, because 21155531 - 2¹⁸ = 20893387 is a prime.

It is a super-2 number, since 2×21155531² = 895112983783922, which contains 22 as substring.

It is a Duffinian number.

It is a junction number, because it is equal to n+sod(n) for n = 21155497 and 21155506.

It is not an unprimeable number, because it can be changed into a prime (21155501) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 32426 + ... + 33071.

It is an arithmetic number, because the mean of its divisors is an integer number (2947410).

Almost surely, 2^21155531 is an apocalyptic number.

21155531 is a deficient number, since it is larger than the sum of its proper divisors (2423749).

21155531 is a wasteful number, since it uses less digits than its factorization.

21155531 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 65533.

The product of its digits is 750, while the sum is 23.

The square root of 21155531 is about 4599.5142134795. The cubic root of 21155531 is about 276.5718513505.

The spelling of 21155531 in words is "twenty-one million, one hundred fifty-five thousand, five hundred thirty-one".