21506451 has 4 divisors (see below), whose sum is σ = 28675272. Its totient is φ = 14337632.

The previous prime is 21506437. The next prime is 21506453. The reversal of 21506451 is 15460512.

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 21506451 - 2⁷ = 21506323 is a prime.

It is not an unprimeable number, because it can be changed into a prime (21506453) 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, 3584406 + ... + 3584411.

It is an arithmetic number, because the mean of its divisors is an integer number (7168818).

Almost surely, 2^21506451 is an apocalyptic number.

21506451 is a deficient number, since it is larger than the sum of its proper divisors (7168821).

21506451 is an equidigital number, since it uses as much as digits as its factorization.

21506451 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 7168820.

The product of its (nonzero) digits is 1200, while the sum is 24.

The square root of 21506451 is about 4637.5048247953. The cubic root of 21506451 is about 278.0926963449.

Adding to 21506451 its reverse (15460512), we get a palindrome (36966963).

The spelling of 21506451 in words is "twenty-one million, five hundred six thousand, four hundred fifty-one".