Income is conventionally denoted by "Y" in economics. John Hicks applied "I" for income, but Keynes wrote to him in 1937, "right after hoping both, I think it is actually easier to use Y for income And that i for expenditure. They'll make a number of deductions from this quantity https://income21093.tinyblogging.com/the-2-minute-rule-for-income-79001902