Television viewer Michael Grigg was stunned to learn Monday that Lovin' Spoonful frontman John Sebastian, not Randy Newman, composed and sang the Welcome Back, Kotter theme song. 'Seriously? John Sebastian?' Grigg said upon being informed by friend Brian Richards while watching the popular '70s sitcom on Nick At Nite. 'God, I would have bet money that it was Randy Newman. Especially with the piano and everything.' Added an incredulous Grigg: 'You have to admit, it sounds a lot more like 'Short People' than 'Do You Believe In Magic?'' 