21005561 has 2 divisors, whose sum is σ = 21005562. Its totient is φ = 21005560.

The previous prime is 21005539. The next prime is 21005597. The reversal of 21005561 is 16550012.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 19669225 + 1336336 = 4435^2 + 1156^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21005561 is a prime.

It is a Chen prime.

It is not a weakly prime, because it can be changed into another prime (21007561) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10502780 + 10502781.

It is an arithmetic number, because the mean of its divisors is an integer number (10502781).

Almost surely, 2^21005561 is an apocalyptic number.

It is an amenable number.

21005561 is a deficient number, since it is larger than the sum of its proper divisors (1).

21005561 is an equidigital number, since it uses as much as digits as its factorization.

21005561 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 300, while the sum is 20.

The square root of 21005561 is about 4583.1824096363. The cubic root of 21005561 is about 275.9167684687.

Adding to 21005561 its reverse (16550012), we get a palindrome (37555573).

The spelling of 21005561 in words is "twenty-one million, five thousand, five hundred sixty-one".