The constant function K = Y*X is presently used by Raydium. This equation has a unique virtue in that it is stateless, meaning that it may supply "unlimited" liquidity to traders given any two tokens and no knowledge about their relative prices or values. Raydium uses this algorithm to price orders on the Serum order book according to the Fibonacci sequence, allowing it to give up to 20 orders at a range of prices with spreads as low as 25BPS, which is ideal for Solana.