21001551 has 4 divisors (see below), whose sum is σ = 28002072. Its totient is φ = 14001032.

The previous prime is 21001549. The next prime is 21001559. The reversal of 21001551 is 15510012.

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 21001551 - 2¹ = 21001549 is a prime.

It is a self number, because there is not a number n which added to its sum of digits gives 21001551.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21001559) 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, 3500256 + ... + 3500261.

It is an arithmetic number, because the mean of its divisors is an integer number (7000518).

Almost surely, 2^21001551 is an apocalyptic number.

21001551 is a deficient number, since it is larger than the sum of its proper divisors (7000521).

21001551 is an equidigital number, since it uses as much as digits as its factorization.

21001551 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 7000520.

The product of its (nonzero) digits is 50, while the sum is 15.

The square root of 21001551 is about 4582.7449198052. The cubic root of 21001551 is about 275.8992096795.

Adding to 21001551 its reverse (15510012), we get a palindrome (36511563).

The spelling of 21001551 in words is "twenty-one million, one thousand, five hundred fifty-one".