1661520721 has 32 divisors (see below), whose sum is σ = 2126304000. Its totient is φ = 1263900672.

The previous prime is 1661520709. The next prime is 1661520787. The reversal of 1661520721 is 1270251661.

It is not a de Polignac number, because 1661520721 - 2⁷ = 1661520593 is a prime.

It is a super-2 number, since 2×1661520721² = 5521302212624719682, which contains 22 as substring.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (1661520221) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 31 ways as a sum of consecutive naturals, for example, 516165 + ... + 519373.

It is an arithmetic number, because the mean of its divisors is an integer number (66447000).

Almost surely, 2^1661520721 is an apocalyptic number.

It is an amenable number.

1661520721 is a deficient number, since it is larger than the sum of its proper divisors (464783279).

1661520721 is a wasteful number, since it uses less digits than its factorization.

1661520721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 3481.

The product of its (nonzero) digits is 5040, while the sum is 31.

The square root of 1661520721 is about 40761.7556172449. The cubic root of 1661520721 is about 1184.4096048359.

The spelling of 1661520721 in words is "one billion, six hundred sixty-one million, five hundred twenty thousand, seven hundred twenty-one".