171

1 + 7 + 1

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

Reads the same in both directions.

Leaves remainder 1 on division by 2.

Composite — it factors as 3^2 × 19.

2 distinct prime factors, 3 with multiplicity.

At least one prime appears squared.

Zero, because a squared prime divides n.

A gap of 6 between them.

1, 3, 9, 19, 57, 171

The proper divisors — everything except n itself.

Its proper divisors fall short by 82.

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.

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

Divisible by its digit sum 9 — 171 ÷ 9 = 19.

Peaks at 9,232 along the way.