155661991 has 4 divisors (see below), whose sum is σ = 156522184. Its totient is φ = 154801800.

The previous prime is 155661983. The next prime is 155662009. The reversal of 155661991 is 199166551.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 199166551 = 83 ⋅2399597.

It is a cyclic number.

It is not a de Polignac number, because 155661991 - 2³ = 155661983 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 (155661931) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 429825 + ... + 430186.

It is an arithmetic number, because the mean of its divisors is an integer number (39130546).

Almost surely, 2^155661991 is an apocalyptic number.

155661991 is a deficient number, since it is larger than the sum of its proper divisors (860193).

155661991 is an equidigital number, since it uses as much as digits as its factorization.

155661991 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 860192.

The product of its digits is 72900, while the sum is 43.

The square root of 155661991 is about 12476.4574699712. The cubic root of 155661991 is about 537.9321821344.

The spelling of 155661991 in words is "one hundred fifty-five million, six hundred sixty-one thousand, nine hundred ninety-one".