166021561 has 2 divisors, whose sum is σ = 166021562. Its totient is φ = 166021560.

The previous prime is 166021543. The next prime is 166021573. The reversal of 166021561 is 165120661.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 118135161 + 47886400 = 10869^2 + 6920^2 .

It is an emirp because it is prime and its reverse (165120661) is a distict prime.

It is a cyclic number.

It is not a de Polignac number, because 166021561 - 2⁹ = 166021049 is a prime.

It is not a weakly prime, because it can be changed into another prime (166021861) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 83010780 + 83010781.

It is an arithmetic number, because the mean of its divisors is an integer number (83010781).

Almost surely, 2^166021561 is an apocalyptic number.

It is an amenable number.

166021561 is a deficient number, since it is larger than the sum of its proper divisors (1).

166021561 is an equidigital number, since it uses as much as digits as its factorization.

166021561 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 2160, while the sum is 28.

The square root of 166021561 is about 12884.9354286314. The cubic root of 166021561 is about 549.6102594248.

The spelling of 166021561 in words is "one hundred sixty-six million, twenty-one thousand, five hundred sixty-one".