21221561 has 2 divisors, whose sum is σ = 21221562. Its totient is φ = 21221560.

The previous prime is 21221539. The next prime is 21221569. The reversal of 21221561 is 16512212.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 20611600 + 609961 = 4540^2 + 781^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21221561 is a prime.

It is not a weakly prime, because it can be changed into another prime (21221569) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10610780 + 10610781.

It is an arithmetic number, because the mean of its divisors is an integer number (10610781).

Almost surely, 2^21221561 is an apocalyptic number.

It is an amenable number.

21221561 is a deficient number, since it is larger than the sum of its proper divisors (1).

21221561 is an equidigital number, since it uses as much as digits as its factorization.

21221561 is an evil number, because the sum of its binary digits is even.

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

The square root of 21221561 is about 4606.6865532615. The cubic root of 21221561 is about 276.8592950547.

Adding to 21221561 its reverse (16512212), we get a palindrome (37733773).

The spelling of 21221561 in words is "twenty-one million, two hundred twenty-one thousand, five hundred sixty-one".