Riemann's Paper — Reading Set

Read the passage below and answer the sub-questions on the right.

In 1859, Bernhard Riemann published an eight-page paper that would haunt mathematicians for generations. The paper contained an offhand conjecture about the zeros of a certain complex function. Its true significance, however, lay not in the calculation itself but in what it revealed about the underlying structure of prime numbers. Even today, more than a century and a half later, that conjecture remains one of the deepest unsolved problems in mathematics — a quiet, obstinate crack running beneath the surface of number theory.

The four sentences below, labeled 1, 2, 3, 4, when properly sequenced form the passage above. Enter the correct order as a 4-digit number (e.g. 3142).

1. However, its true significance lies not in the calculation itself but in what it revealed about the underlying structure of prime numbers.
2. In 1859, Bernhard Riemann published an eight-page paper that would haunt mathematicians for generations.
3. The paper contained an offhand conjecture about the zeros of a certain complex function.
4. Even today, more than a century and a half later, that conjecture remains one of the deepest unsolved problems in mathematics.

The author's primary purpose in the passage is to:

The tone of the closing sentence ("a quiet, obstinate crack…") is best described as: