• 19 can be written using four 4's:

19 has 2 divisors, whose sum is σ = 20. Its totient is φ = 18.

The previous prime is 17. The next prime is 23. The reversal of 19 is 91.

19 = T_{2} + T_{3} +
T_{4}.

It is a happy number.

It is an alternating factorial (19 = 4! - 3! + 2! - 1!).

19 is digitally balanced in base 3, because in such base it contains all the possibile digits an equal number of times.

19 is an esthetic number in base 5, base 8 and base 9, because in such bases its adjacent digits differ by 1.

It is a weak prime.

It is a cyclic number.

It is not a de Polignac number, because 19 - 2^{1} = 17 is a prime.

It is a super-2 number, since 2×19^{2} = 722, which contains 22 as substring.

Together with 17, it forms a pair of twin primes.

It is a Chen prime.

19 is a repfigit number.

It is a fibodiv number, since the Fibonacci-like sequence with seeds 1 and 9 contains 19 itself.

19 is a modest number, since divided by 9 gives 1 as remainder.

19 is strictly pandigital in base 3.

It is a plaindrome in base 5, base 7, base 8, base 10, base 11, base 12, base 13, base 14, base 15 and base 16.

It is a nialpdrome in base 6 and base 9.

It is a pernicious number, because its binary representation contains a prime number (3) of ones.

It is an upside-down number.

It is a Pierpont prime, being equal to 2^{1} ⋅ 3^{2} + 1.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 9 + 10.

It is an arithmetic number, because the mean of its divisors is an integer number (10).

19 is the 4-th centered triangular number and also the 3-rd hex number.

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

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

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

The product of its digits is 9, while the sum is 10.

The square root of 19 is about 4.3588989435. The cubic root of 19 is about 2.6684016487.

Multiplying 19 by its sum of digits (10), we get a triangular number (190 = T_{19}).

The spelling of 19 in words is "nineteen", and thus it is an aban number, an oban number, and an uban number.

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