21663251 has 2 divisors, whose sum is σ = 21663252. Its totient is φ = 21663250.

The previous prime is 21663247. The next prime is 21663277. The reversal of 21663251 is 15236612.

It is an a-pointer prime, because the next prime (21663277) can be obtained adding 21663251 to its sum of digits (26).

It is a weak prime.

It is a cyclic number.

It is not a de Polignac number, because 21663251 - 2² = 21663247 is a prime.

It is a Chen prime.

It is a self number, because there is not a number n which added to its sum of digits gives 21663251.

It is not a weakly prime, because it can be changed into another prime (21663281) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10831625 + 10831626.

It is an arithmetic number, because the mean of its divisors is an integer number (10831626).

Almost surely, 2^21663251 is an apocalyptic number.

21663251 is a deficient number, since it is larger than the sum of its proper divisors (1).

21663251 is an equidigital number, since it uses as much as digits as its factorization.

21663251 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 2160, while the sum is 26.

The square root of 21663251 is about 4654.3797653393. The cubic root of 21663251 is about 278.7669031959.

Adding to 21663251 its reverse (15236612), we get a palindrome (36899863).

The spelling of 21663251 in words is "twenty-one million, six hundred sixty-three thousand, two hundred fifty-one".