1661221555 has 16 divisors (see below), whose sum is σ = 2412262080. Its totient is φ = 1072115712.

The previous prime is 1661221531. The next prime is 1661221561. The reversal of 1661221555 is 5551221661.

It is a cyclic number.

It is not a de Polignac number, because 1661221555 - 2⁷ = 1661221427 is a prime.

It is a self number, because there is not a number n which added to its sum of digits gives 1661221555.

It is an unprimeable number.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 1395390 + ... + 1396579.

It is an arithmetic number, because the mean of its divisors is an integer number (150766380).

Almost surely, 2^1661221555 is an apocalyptic number.

1661221555 is a deficient number, since it is larger than the sum of its proper divisors (751040525).

1661221555 is a wasteful number, since it uses less digits than its factorization.

1661221555 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 2791998.

The product of its digits is 18000, while the sum is 34.

The square root of 1661221555 is about 40758.0857622141. The cubic root of 1661221555 is about 1184.3385140678.

The spelling of 1661221555 in words is "one billion, six hundred sixty-one million, two hundred twenty-one thousand, five hundred fifty-five".