21717251 has 4 divisors (see below), whose sum is σ = 22085400. Its totient is φ = 21349104.

The previous prime is 21717217. The next prime is 21717253. The reversal of 21717251 is 15271712.

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 21717251 - 2¹⁸ = 21455107 is a prime.

It is a Duffinian number.

It is a self number, because there is not a number n which added to its sum of digits gives 21717251.

It is not an unprimeable number, because it can be changed into a prime (21717253) 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, 183986 + ... + 184103.

It is an arithmetic number, because the mean of its divisors is an integer number (5521350).

Almost surely, 2^21717251 is an apocalyptic number.

21717251 is a deficient number, since it is larger than the sum of its proper divisors (368149).

21717251 is an equidigital number, since it uses as much as digits as its factorization.

21717251 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 368148.

The product of its digits is 980, while the sum is 26.

The square root of 21717251 is about 4660.1771425558. The cubic root of 21717251 is about 278.9983384835.

Adding to 21717251 its reverse (15271712), we get a palindrome (36988963).

The spelling of 21717251 in words is "twenty-one million, seven hundred seventeen thousand, two hundred fifty-one".