21215561 has 2 divisors, whose sum is σ = 21215562. Its totient is φ = 21215560.

The previous prime is 21215543. The next prime is 21215569. The reversal of 21215561 is 16551212.

It is a happy number.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 15721225 + 5494336 = 3965^2 + 2344^2 .

It is a cyclic number.

It is not a de Polignac number, because 21215561 - 2⁶ = 21215497 is a prime.

It is a Chen prime.

It is not a weakly prime, because it can be changed into another prime (21215569) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10607780 + 10607781.

It is an arithmetic number, because the mean of its divisors is an integer number (10607781).

Almost surely, 2^21215561 is an apocalyptic number.

It is an amenable number.

21215561 is a deficient number, since it is larger than the sum of its proper divisors (1).

21215561 is an equidigital number, since it uses as much as digits as its factorization.

21215561 is an evil number, because the sum of its binary digits is even.

The product of its digits is 600, while the sum is 23.

The square root of 21215561 is about 4606.0352799344. The cubic root of 21215561 is about 276.8332003304.

Adding to 21215561 its reverse (16551212), we get a palindrome (37766773).

The spelling of 21215561 in words is "twenty-one million, two hundred fifteen thousand, five hundred sixty-one".