Double-struck capital N

Blackboard-bold N, the standard symbol for the set of natural numbers. Note the convention trap: some authors include 0 in ℕ and others start at 1, so papers often clarify with ℕ₀ or ℕ⁺. Encoded as a standalone character, it pastes cleanly into plain text alongside ℤ, ℚ, and ℝ.