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Had a Jane Street interview in 2013 that still bothers me. It was my 6th round. Final interview. The guy walks in carrying no laptop, no notebook, just a cold brew and what I later realized was a single IKEA tea candle. He writes on the whiteboard: food: $200 rent: $800 utilities: $150 candles: $3,600 family: dying Then he turns around and says, “Optimize.” I laughed because I thought it was a culture-fit bit. He did not laugh. So I said, “Well, obviously you spend less on candles.” He says, “Assume candles are non-discretionary.” Okay. I start building a model. Basic constraint satisfaction. Family survival as a soft penalty. Candles as a state variable. Maybe there’s an arbitrage where you buy wholesale paraffin and convert the $3,600 line item into inventory. He stops me. “You’re thinking like a consultant.” That’s when I knew I was in trouble. He says, “Give me a bid-ask on family dying.” I say, “What?” He says, “You’re long candles, short family. Where do you make markets?” I try to recover. I say the real issue is liquidity: rent and utilities are fixed, food is elastic, candles are emotionally inelastic. Therefore the optimal strategy is to securitize future candle enjoyment and borrow against it. He nods for the first time. Then he asks, “What time do you sell the candles?” I say, “Whenever the market is liquid?” He says, “Be more specific.” I say, “Uh… 10 a.m. Eastern?” For the first time, he smiles. He goes, “Every day?” I say, “Every day.” He says, “In size?” I say, “In size.” He says, “And what do we call that?” I say, “Market manipulation?” The room gets very quiet. He looks disappointed and writes something down. “No. We call it providing liquidity to candle ETFs during the U.S. cash open.” I try to save it. “Right. Of course. The family isn’t dying because we underfunded them. They’re just experiencing temporary price discovery.” He nods again. Then he points back at the board. I had missed it. The utility bill was $150, but candles provide light. You can zero out utilities. I update the budget: food: $200 rent: $800 utilities: $0 candles: $3,750 family: still dying, but now in a more capital-efficient way He says, “How confident are you?” I say, “0.95.” He smiles and circles candles. “0.95 huh?” Then he asks me to estimate how many leveraged longs get liquidated if we dump $3,750 of candles at 10:00:01 every morning for 90 consecutive trading days. Needless to say I did not get the offer.

kalman filter