21115551 has 8 divisors (see below), whose sum is σ = 28300720. Its totient is φ = 14003712.

The previous prime is 21115511. The next prime is 21115559. The reversal of 21115551 is 15551112.

It is a sphenic number, since it is the product of 3 distinct primes.

It is not a de Polignac number, because 21115551 - 2¹⁵ = 21082783 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 (21115559) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 17656 + ... + 18813.

It is an arithmetic number, because the mean of its divisors is an integer number (3537590).

Almost surely, 2^21115551 is an apocalyptic number.

21115551 is a deficient number, since it is larger than the sum of its proper divisors (7185169).

21115551 is a wasteful number, since it uses less digits than its factorization.

21115551 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 36665.

The product of its digits is 250, while the sum is 21.

The square root of 21115551 is about 4595.1660470542. The cubic root of 21115551 is about 276.3975184479.

Adding to 21115551 its reverse (15551112), we get a palindrome (36666663).

The spelling of 21115551 in words is "twenty-one million, one hundred fifteen thousand, five hundred fifty-one".