38

3 + 8

Digit sum repeated until one digit remains; equals n mod 9 (with 9 for multiples of 9).

Reverses to 83.

Divisible by 2.

Composite — it factors as 2 × 19.

2 distinct prime factors, 2 with multiplicity.

No prime divides it twice.

A product of exactly two primes, counted with multiplicity.

(−1) to the power of 2 distinct prime factors.

A gap of 4 between them.

1, 2, 19, 38

The proper divisors — everything except n itself.

Its proper divisors fall short by 16.

How many integers from 1 to n share no factor with n.

A prime congruent to 3 mod 4 divides it an odd number of times, which rules it out.

Legendre's three-square theorem allows it.

Lagrange's four-square theorem: every non-negative integer is, without exception.

The smallest pair of primes summing to n. Goldbach’s conjecture says one always exists; it remains unproven.

Not triangular, square, pentagonal, hexagonal, cubic, Fibonacci, Lucas, Catalan, factorial or a power of two.

Summing the squares of its digits falls into the 4 → 16 → 37 → … → 4 cycle.

Not divisible by its digit sum 11.

Peaks at 88 along the way.