21455551 has 4 divisors (see below), whose sum is σ = 23105992. Its totient is φ = 19805112.

The previous prime is 21455549. The next prime is 21455557. The reversal of 21455551 is 15555412.

It is a semiprime because it is the product of two primes.

It is a cyclic number.

It is not a de Polignac number, because 21455551 - 2¹ = 21455549 is a prime.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21455557) 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, 825201 + ... + 825226.

It is an arithmetic number, because the mean of its divisors is an integer number (5776498).

Almost surely, 2^21455551 is an apocalyptic number.

21455551 is a deficient number, since it is larger than the sum of its proper divisors (1650441).

21455551 is a wasteful number, since it uses less digits than its factorization.

21455551 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1650440.

The product of its digits is 5000, while the sum is 28.

The square root of 21455551 is about 4632.0137089607. The cubic root of 21455551 is about 277.8731327682.

It can be divided in two parts, 2145555 and 1, that added together give a triangular number (2145556 = T₂₀₇₁).

The spelling of 21455551 in words is "twenty-one million, four hundred fifty-five thousand, five hundred fifty-one".