21506551 has 4 divisors (see below), whose sum is σ = 23461704. Its totient is φ = 19551400.

The previous prime is 21506533. The next prime is 21506593. The reversal of 21506551 is 15560512.

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

It is not a de Polignac number, because 21506551 - 2⁵ = 21506519 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21536551) 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 3 ways as a sum of consecutive naturals, for example, 977560 + ... + 977581.

It is an arithmetic number, because the mean of its divisors is an integer number (5865426).

Almost surely, 2^21506551 is an apocalyptic number.

21506551 is a deficient number, since it is larger than the sum of its proper divisors (1955153).

21506551 is a wasteful number, since it uses less digits than its factorization.

21506551 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1955152.

The product of its (nonzero) digits is 1500, while the sum is 25.

The square root of 21506551 is about 4637.5156064427. The cubic root of 21506551 is about 278.0931273664.

The spelling of 21506551 in words is "twenty-one million, five hundred six thousand, five hundred fifty-one".