21155661 has 20 divisors (see below), whose sum is σ = 31847200. Its totient is φ = 13994856.

The previous prime is 21155627. The next prime is 21155663. The reversal of 21155661 is 16655112.

It is a happy number.

It is not a de Polignac number, because 21155661 - 2⁹ = 21155149 is a prime.

It is a Harshad number since it is a multiple of its sum of digits (27).

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21155663) 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 19 ways as a sum of consecutive naturals, for example, 10320 + ... + 12198.

It is an arithmetic number, because the mean of its divisors is an integer number (1592360).

Almost surely, 2^21155661 is an apocalyptic number.

It is an amenable number.

21155661 is a deficient number, since it is larger than the sum of its proper divisors (10691539).

21155661 is a wasteful number, since it uses less digits than its factorization.

21155661 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 2030 (or 2021 counting only the distinct ones).

The product of its digits is 1800, while the sum is 27.

The square root of 21155661 is about 4599.5283453850. The cubic root of 21155661 is about 276.5724178575.

It can be divided in two parts, 2115 and 5661, that added together give a 5-th power (7776 = 6⁵).

The spelling of 21155661 in words is "twenty-one million, one hundred fifty-five thousand, six hundred sixty-one".