1263991991 has 4 divisors (see below), whose sum is σ = 1270408392. Its totient is φ = 1257575592.

The previous prime is 1263991957. The next prime is 1263992029. The reversal of 1263991991 is 1991993621.

It is a happy number.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 1991993621 = 47 ⋅42382843.

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-1263991991 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 (1263991591) 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, 3207905 + ... + 3208298.

It is an arithmetic number, because the mean of its divisors is an integer number (317602098).

Almost surely, 2^1263991991 is an apocalyptic number.

1263991991 is a deficient number, since it is larger than the sum of its proper divisors (6416401).

1263991991 is an equidigital number, since it uses as much as digits as its factorization.

1263991991 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 6416400.

The product of its digits is 236196, while the sum is 50.

The square root of 1263991991 is about 35552.6650337215. The cubic root of 1263991991 is about 1081.2217515157.

The spelling of 1263991991 in words is "one billion, two hundred sixty-three million, nine hundred ninety-one thousand, nine hundred ninety-one".