"So - this is what the professor was working on before he died?" Inspector Obata indicated the whiteboard that dominated the dead man's office. The surface of the whiteboard was covered in mathematical signs and symbols that had been drawn in erasable marker. Streaks of black showed where strings of digits had been wiped away, then replaced.

"Hai, keibuho-san," Izumi Naoko - Professor Odawara's senior student - bowed to the detective. "It is a derivation of - ."

The inspector waved a hand at the young lady. "I doubt the subject of the professor's work will have any bearing on this investigation. I am merely here to determine whether there was any foul play involved with his death.

"Now, tell me, was the professor passionate about his work?"

"Very. Odawara-sensei would often work all night on his theory."

"Indeed." Inspector Obata stirred a pile of discarded papers that covered the professor's desk, but the abstruse mathematics gave him no insights. "Was his housekeeping always this bad?"

Naoko shook her head. "No sir. It was never good, but," she took a deep breath, "for the last month he locked his office. Nobody was allowed in - not us students, and definitely not the cleaners."

Obata nodded. "I see. And the professor's behaviour over this last month? Would you describe it as erratic?"

"He was not his usual self. He neglected his lectures and cancelled his tutorials. Sometimes he would even forget to eat."

"I see." The inspector thought for a moment. The professor's death had all the hallmarks of karōshi - death by overwork. "Thank you, Izumi-san. There are no more questions for now. If you will excuse me." Obata bowed and left the office.

Naoko sat down on the professor's desk. Something about the whiteboard caught her eye. There was something in the lines of symbols that bothered her - something wrong.

"If I change this variable," Naoko thought, "it should make more sense."

She reached for a marker pen and began to scribble.