On the Waring-Goldbach Problem for Fourth and Fifth Powers

It is shown that every sufficiently large integer congruentto 14 modulo 240 may be written as the sum of 14 fourth powersof prime numbers, and that every sufficiently large odd integermay be written as the sum of 21 fifth powers of prime numbers.The respective implicit bounds 14 and 21 improve on the previousbounds 15 (following from work of Davenport) and 23 (due toThanigasalam). These conclusions are established through themedium of the Hardy-Littlewood method, the proofs being somewhatnovel in their use of estimates stemming directly from exponentialsums over prime numbers in combination with the linear sieve,rather than the conventional methods which ‘waste’a variable or two by throwing minor arc estimates down to anauxiliary mean value estimate based on variables not restrictedto be prime numbers. In the work on fifth powers, a switchingprinciple is applied to a cognate problem involving almost primesin order to obtain the desired conclusion involving prime numbersalone. 2000 Mathematics Subject Classification: 11P05, 11N36,11L15, 11P55.