104

1 + 0 + 4

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

Reverses to 401.

Divisible by 2.

Composite — it factors as 2^3 × 13.

2 distinct prime factors, 4 with multiplicity.

At least one prime appears squared.

Zero, because a squared prime divides n.

A gap of 4 between them.

1, 2, 4, 8, 13, 26, 52, 104

The proper divisors — everything except n itself.

Its proper divisors overshoot by 2.

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

2² + 10² = 104

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 5.

Peaks at 104 along the way.