The Professor handed her the chalk.
One student whispered:
"This is going to be brutal."
The Black woman heard.
She turned toward the board.
For perhaps ten seconds, she did nothing.
Then she raised the chalk.
Not toward the top.
Toward the third line.
She drew a vertical mark beside one term.
The Professor smiled.
"Starting in the middle?"
She ignored him.
She circled a matrix expression.
Then crossed out one subscript.
The Professor's smile faded slightly.
"What are you doing?"
"Fixing your transcription error."
The room reacted.
He stepped closer.
"There is no transcription error."
She pointed.
"This index should be j minus one."
"No."
"Yes."
He laughed.
"Based on what?"
She looked at him.
"The recurrence doesn't close otherwise."
A graduate student immediately began typing.
Another opened a notebook.
The Professor's face tightened.
"The formulation permits—"
"No."
She interrupted calmly.
"It doesn't."
The interruption shocked the room more than the claim.
She continued.
"If you leave it as j, the conditional operator on the next line includes a term you already conditioned away."
She pointed toward another expression.
"Then this covariance becomes singular."
Several graduate students leaned forward.
One whispered:
"She's right."
The Professor heard.
His jaw tightened.
The Black woman erased the subscript.
Rewrote it.
Then stepped back.
"Now it works."
The Professor crossed his arms.
"Fine."
He smiled again.
"One correction."
"Forty-nine thousand nine hundred dollars to go."
A few nervous laughs.
The woman looked at him.
"No."
She tapped the board.
"This is worth more than that."
"What?"