66991021 has 2 divisors, whose sum is σ = 66991022. Its totient is φ = 66991020.

The previous prime is 66990997. The next prime is 66991037. The reversal of 66991021 is 12019966.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 39312900 + 27678121 = 6270^2 + 5261^2 .

It is a cyclic number.

It is not a de Polignac number, because 66991021 - 2⁷ = 66990893 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (66991051) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 33495510 + 33495511.

It is an arithmetic number, because the mean of its divisors is an integer number (33495511).

Almost surely, 2^66991021 is an apocalyptic number.

It is an amenable number.

66991021 is a deficient number, since it is larger than the sum of its proper divisors (1).

66991021 is an equidigital number, since it uses as much as digits as its factorization.

66991021 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 5832, while the sum is 34.

The square root of 66991021 is about 8184.8042737747. The cubic root of 66991021 is about 406.1366656318.

The spelling of 66991021 in words is "sixty-six million, nine hundred ninety-one thousand, twenty-one".