1656721621 has 8 divisors (see below), whose sum is σ = 1757144664. Its totient is φ = 1556628480.

The previous prime is 1656721609. The next prime is 1656721637. The reversal of 1656721621 is 1261276561.

It is a happy number.

It can be written as a sum of positive squares in 4 ways, for example, as 530380900 + 1126340721 = 23030^2 + 33561^2 .

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

It is a cyclic number.

It is not a de Polignac number, because 1656721621 - 2¹⁷ = 1656590549 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 (1656721601) 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, 72090 + ... + 92251.

It is an arithmetic number, because the mean of its divisors is an integer number (219643083).

Almost surely, 2^1656721621 is an apocalyptic number.

It is an amenable number.

1656721621 is a deficient number, since it is larger than the sum of its proper divisors (100423043).

1656721621 is a wasteful number, since it uses less digits than its factorization.

1656721621 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 164951.

The product of its digits is 30240, while the sum is 37.

The square root of 1656721621 is about 40702.8453673696. The cubic root of 1656721621 is about 1183.2681642525.

The spelling of 1656721621 in words is "one billion, six hundred fifty-six million, seven hundred twenty-one thousand, six hundred twenty-one".