66991681 has 4 divisors (see below), whose sum is σ = 67023792. Its totient is φ = 66959572.

The previous prime is 66991669. The next prime is 66991693. The reversal of 66991681 is 18619966.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4, and also an emirpimes, since its reverse is a distinct semiprime: 18619966 = 2 ⋅9309983.

It is an interprime number because it is at equal distance from previous prime (66991669) and next prime (66991693).

It is a cyclic number.

It is not a de Polignac number, because 66991681 - 2¹¹ = 66989633 is a prime.

It is a super-2 number, since 2×66991681² = 8975770646411522, which contains 22 as substring.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (66991601) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 12691 + ... + 17176.

It is an arithmetic number, because the mean of its divisors is an integer number (16755948).

Almost surely, 2^66991681 is an apocalyptic number.

It is an amenable number.

66991681 is a deficient number, since it is larger than the sum of its proper divisors (32111).

66991681 is a wasteful number, since it uses less digits than its factorization.

66991681 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 32110.

The product of its digits is 139968, while the sum is 46.

The square root of 66991681 is about 8184.8445922937. The cubic root of 66991681 is about 406.1379993893.

The spelling of 66991681 in words is "sixty-six million, nine hundred ninety-one thousand, six hundred eighty-one".