20721566 has 4 divisors (see below), whose sum is σ = 31082352. Its totient is φ = 10360782.

The previous prime is 20721553. The next prime is 20721583. The reversal of 20721566 is 66512702.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 66512702 = 2 ⋅33256351.

It is a congruent number.

It is an unprimeable number.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 5180390 + ... + 5180393.

It is an arithmetic number, because the mean of its divisors is an integer number (7770588).

Almost surely, 2^20721566 is an apocalyptic number.

20721566 is a deficient number, since it is larger than the sum of its proper divisors (10360786).

20721566 is a wasteful number, since it uses less digits than its factorization.

20721566 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 10360785.

The product of its (nonzero) digits is 5040, while the sum is 29.

The square root of 20721566 is about 4552.0946826708. The cubic root of 20721566 is about 274.6676580252.

The spelling of 20721566 in words is "twenty million, seven hundred twenty-one thousand, five hundred sixty-six".