1661155781 has 2 divisors, whose sum is σ = 1661155782. Its totient is φ = 1661155780.

The previous prime is 1661155751. The next prime is 1661155789. The reversal of 1661155781 is 1875511661.

It is a happy number.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 1311091681 + 350064100 = 36209^2 + 18710^2 .

It is a cyclic number.

It is not a de Polignac number, because 1661155781 - 2¹⁴ = 1661139397 is a prime.

It is a super-2 number, since 2×1661155781² = 5518877057499439922, which contains 22 as substring.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (1661155789) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 830577890 + 830577891.

It is an arithmetic number, because the mean of its divisors is an integer number (830577891).

Almost surely, 2^1661155781 is an apocalyptic number.

It is an amenable number.

1661155781 is a deficient number, since it is larger than the sum of its proper divisors (1).

1661155781 is an equidigital number, since it uses as much as digits as its factorization.

1661155781 is an evil number, because the sum of its binary digits is even.

The product of its digits is 50400, while the sum is 41.

The square root of 1661155781 is about 40757.2788713869. The cubic root of 1661155781 is about 1184.3228830584.

The spelling of 1661155781 in words is "one billion, six hundred sixty-one million, one hundred fifty-five thousand, seven hundred eighty-one".