21217851 has 24 divisors (see below), whose sum is σ = 32497920. Its totient is φ = 13302576.

The previous prime is 21217849. The next prime is 21217907. The reversal of 21217851 is 15871212.

It is not a de Polignac number, because 21217851 - 2¹ = 21217849 is a prime.

It is not an unprimeable number, because it can be changed into a prime (21217151) by changing a digit.

It is a polite number, since it can be written in 23 ways as a sum of consecutive naturals, for example, 28186 + ... + 28928.

It is an arithmetic number, because the mean of its divisors is an integer number (1354080).

Almost surely, 2^21217851 is an apocalyptic number.

21217851 is a deficient number, since it is larger than the sum of its proper divisors (11280069).

21217851 is a wasteful number, since it uses less digits than its factorization.

21217851 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 935 (or 932 counting only the distinct ones).

The product of its digits is 1120, while the sum is 27.

The square root of 21217851 is about 4606.2838601198. The cubic root of 21217851 is about 276.8431603973.

The spelling of 21217851 in words is "twenty-one million, two hundred seventeen thousand, eight hundred fifty-one".