20981561 has 2 divisors, whose sum is σ = 20981562. Its totient is φ = 20981560.

The previous prime is 20981557. The next prime is 20981573. The reversal of 20981561 is 16518902.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 19404025 + 1577536 = 4405^2 + 1256^2 .

It is a cyclic number.

It is not a de Polignac number, because 20981561 - 2² = 20981557 is a prime.

It is a Chen prime.

It is not a weakly prime, because it can be changed into another prime (20981551) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10490780 + 10490781.

It is an arithmetic number, because the mean of its divisors is an integer number (10490781).

Almost surely, 2^20981561 is an apocalyptic number.

It is an amenable number.

20981561 is a deficient number, since it is larger than the sum of its proper divisors (1).

20981561 is an equidigital number, since it uses as much as digits as its factorization.

20981561 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 4320, while the sum is 32.

The square root of 20981561 is about 4580.5633932956. The cubic root of 20981561 is about 275.8116450993.

The spelling of 20981561 in words is "twenty million, nine hundred eighty-one thousand, five hundred sixty-one".