19891661 has 2 divisors, whose sum is σ = 19891662. Its totient is φ = 19891660.

The previous prime is 19891657. The next prime is 19891667. The reversal of 19891661 is 16619891.

It is a happy number.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 18748900 + 1142761 = 4330^2 + 1069^2 .

It is an emirp because it is prime and its reverse (16619891) is a distict prime.

It is a cyclic number.

It is not a de Polignac number, because 19891661 - 2² = 19891657 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (19891667) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 9945830 + 9945831.

It is an arithmetic number, because the mean of its divisors is an integer number (9945831).

Almost surely, 2^19891661 is an apocalyptic number.

It is an amenable number.

19891661 is a deficient number, since it is larger than the sum of its proper divisors (1).

19891661 is an equidigital number, since it uses as much as digits as its factorization.

19891661 is an evil number, because the sum of its binary digits is even.

The product of its digits is 23328, while the sum is 41.

The square root of 19891661 is about 4460.0068385598. The cubic root of 19891661 is about 270.9507451685.

The spelling of 19891661 in words is "nineteen million, eight hundred ninety-one thousand, six hundred sixty-one".