21561551 has 4 divisors (see below), whose sum is σ = 23521704. Its totient is φ = 19601400.

The previous prime is 21561503. The next prime is 21561571. The reversal of 21561551 is 15516512.

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 21561551 - 2¹⁸ = 21299407 is a prime.

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 21561551.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21561571) 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, 980060 + ... + 980081.

It is an arithmetic number, because the mean of its divisors is an integer number (5880426).

Almost surely, 2^21561551 is an apocalyptic number.

21561551 is a deficient number, since it is larger than the sum of its proper divisors (1960153).

21561551 is a wasteful number, since it uses less digits than its factorization.

21561551 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1960152.

The product of its digits is 1500, while the sum is 26.

The square root of 21561551 is about 4643.4417192423. The cubic root of 21561551 is about 278.3299870116.

The spelling of 21561551 in words is "twenty-one million, five hundred sixty-one thousand, five hundred fifty-one".