21651221 has 2 divisors, whose sum is σ = 21651222. Its totient is φ = 21651220.

The previous prime is 21651209. The next prime is 21651271. The reversal of 21651221 is 12215612.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 12567025 + 9084196 = 3545^2 + 3014^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21651221 is a prime.

It is a Chen prime.

It is a junction number, because it is equal to n+sod(n) for n = 21651193 and 21651202.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21651271) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10825610 + 10825611.

It is an arithmetic number, because the mean of its divisors is an integer number (10825611).

Almost surely, 2^21651221 is an apocalyptic number.

It is an amenable number.

21651221 is a deficient number, since it is larger than the sum of its proper divisors (1).

21651221 is an equidigital number, since it uses as much as digits as its factorization.

21651221 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 240, while the sum is 20.

The square root of 21651221 is about 4653.0872547159. The cubic root of 21651221 is about 278.7152921857.

Adding to 21651221 its reverse (12215612), we get a palindrome (33866833).

It can be divided in two parts, 21651 and 221, that multiplied together give a triangular number (4784871 = T₃₀₉₃).

The spelling of 21651221 in words is "twenty-one million, six hundred fifty-one thousand, two hundred twenty-one".