"The error."
His expression shifted.
"What are you talking about?"
She looked toward the audience.
Then back.
"If someone copied this equation from handwritten notes, j and j minus one are easy to confuse."
The Professor became very still.
"Are you accusing me of copying something?"
"I didn't accuse you of anything."
"You implied it."
"I said the error looks copied."
A student near the front whispered:
"Whoa."
The Professor stepped closer.
"Write."
The woman nodded.
"Gladly."
And she did.
She began at the left edge.
At first, slowly.
One line.
Then another.
She defined a transformation that none of the students had attempted over the weekend.
A doctoral candidate leaned forward.
His eyes widened.
The Professor interrupted.
"That's unnecessary."
She kept writing.
He said louder:
"I said that transformation is unnecessary."
She stopped.
Turned.
"It's the only clean way to separate the stochastic term from the boundary term."
The Professor scoffed.
"According to you."
"According to the equation."
She turned back.
Continued.
Five minutes passed.
Then eight.
Then twelve.
The mood in the room changed completely.
Nobody looked amused anymore.
Students were photographing every line.
Graduate students were working beside her, trying to verify each step before she finished the next.
The Professor began pacing.
The Black woman never looked toward him.
She filled half the remaining board.
Then returned to the earlier recurrence.
Substituted the transformed variable.
Canceled two terms.
Reduced the expression.
A student near the front suddenly said:
"Oh my God."
The Professor spun.
"What?"
The student looked embarrassed.
"Nothing."
"What?"
The doctoral candidate beside him answered instead.
"She found the invariant."