"There's an error on the board."
The room went quiet. Walsh's smile froze. "Excuse me?"
Whitney turned to face him. Her voice did not rise. "Third line, the middle term. You have a plus sign." She paused. "It should be a minus."
Two hundred faces turned to the equation. Most of them couldn't read the symbols, but they could read Walsh's expression.
His jaw tightened. "A stylistic choice. The structure stands."
"No, sir, it doesn't."
Whitney walked over to where the chalk had landed on the floor. She bent down, picked it up, and held it in her hand. She did not walk back to the board—not yet. She faced the room instead.
"The equation Professor Walsh wrote belongs to a class of problems in modular form theory. Every problem in that class has a standard structural form." A pause. "In the standard form, the third term carries a minus, not a plus. It is not a stylistic choice; it is a load-bearing sign."
She turned to Walsh. "With your plus sign, the left side of the equation factors out completely. It collapses into a trivial identity."
"Trivial?" Walsh's voice was sharp. "Now you're telling me—"
"I'm telling you that, as written, your equation isn't a hard problem. It isn't a problem at all. It says zero equals zero."
A laugh broke out from somewhere in the back row: short, sharp. Then it was gone. Sullivan put a hand over her mouth.
Walsh stepped forward, his voice low. "Now I'd like to see you demonstrate that claim on the board. Now."
"Of course."
Whitney walked to the second blackboard. She picked up the chalk. She did not hurry.