81781666 has 4 divisors (see below), whose sum is σ = 122672502. Its totient is φ = 40890832.

The previous prime is 81781639. The next prime is 81781697. The reversal of 81781666 is 66618718.

It is a semiprime because it is the product of two primes.

It can be written as a sum of positive squares in only one way, i.e., 81378441 + 403225 = 9021^2 + 635^2 .

It is an unprimeable number.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 20445415 + ... + 20445418.

Almost surely, 2^81781666 is an apocalyptic number.

81781666 is a deficient number, since it is larger than the sum of its proper divisors (40890836).

81781666 is a wasteful number, since it uses less digits than its factorization.

81781666 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 40890835.

The product of its digits is 96768, while the sum is 43.

The square root of 81781666 is about 9043.3216242706. The cubic root of 81781666 is about 434.0622172194.

The spelling of 81781666 in words is "eighty-one million, seven hundred eighty-one thousand, six hundred sixty-six".