21005551 has 8 divisors (see below), whose sum is σ = 24051104. Its totient is φ = 17971200.

The previous prime is 21005539. The next prime is 21005561. The reversal of 21005551 is 15550012.

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 21005551 - 2⁷ = 21005423 is a prime.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21005561) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 1711 + ... + 6703.

It is an arithmetic number, because the mean of its divisors is an integer number (3006388).

Almost surely, 2^21005551 is an apocalyptic number.

21005551 is a deficient number, since it is larger than the sum of its proper divisors (3045553).

21005551 is an equidigital number, since it uses as much as digits as its factorization.

21005551 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 5601.

The product of its (nonzero) digits is 250, while the sum is 19.

The square root of 21005551 is about 4583.1813186912. The cubic root of 21005551 is about 275.9167246840.

Adding to 21005551 its reverse (15550012), we get a palindrome (36555563).

It can be divided in two parts, 2 and 1005551, that multiplied together give a palindrome (2011102).

The spelling of 21005551 in words is "twenty-one million, five thousand, five hundred fifty-one".