66688021 has 2 divisors, whose sum is σ = 66688022. Its totient is φ = 66688020.

The previous prime is 66688009. The next prime is 66688031. The reversal of 66688021 is 12088666.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 66178225 + 509796 = 8135^2 + 714^2 .

It is a cyclic number.

It is not a de Polignac number, because 66688021 - 2⁵ = 66687989 is a prime.

It is a super-3 number, since 3×66688021³ (a number of 24 digits) contains 333 as substring.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (66688001) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 33344010 + 33344011.

It is an arithmetic number, because the mean of its divisors is an integer number (33344011).

Almost surely, 2^66688021 is an apocalyptic number.

It is an amenable number.

66688021 is a deficient number, since it is larger than the sum of its proper divisors (1).

66688021 is an equidigital number, since it uses as much as digits as its factorization.

66688021 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 27648, while the sum is 37.

The square root of 66688021 is about 8166.2733850882. The cubic root of 66688021 is about 405.5234222061.

The spelling of 66688021 in words is "sixty-six million, six hundred eighty-eight thousand, twenty-one".