661021391 has 8 divisors (see below), whose sum is σ = 680162304. Its totient is φ = 641947680.

The previous prime is 661021373. The next prime is 661021399. The reversal of 661021391 is 193120166.

It is a sphenic number, since it is the product of 3 distinct primes.

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-661021391 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (661021399) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (17) of ones.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 3506 + ... + 36528.

It is an arithmetic number, because the mean of its divisors is an integer number (85020288).

Almost surely, 2^661021391 is an apocalyptic number.

661021391 is a deficient number, since it is larger than the sum of its proper divisors (19140913).

661021391 is a wasteful number, since it uses less digits than its factorization.

661021391 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 33601.

The product of its (nonzero) digits is 1944, while the sum is 29.

The square root of 661021391 is about 25710.3362677348. The cubic root of 661021391 is about 871.1076704732.

The spelling of 661021391 in words is "six hundred sixty-one million, twenty-one thousand, three hundred ninety-one".