Units of paper quantity of 480 sheets, now known as a 'short' ream. Reams of 472 and 516 sheets are still current, but in retail outlets paper is typically sold in reams of 500. As an old UK and US unit, a perfect ream was equal to 516 sheets. Certain types of specialist papers such as tissue paper, greaseproof paper, handmade paper, and blotting paper are still sold (especially in the UK) in 'short' reams of 480 sheets (20 quires of 24 sheets). However, the commercial use of the word 'ream' for quantities of paper other than 500 is now deprecated by such standards
Many-one reduction in: We say that a class C of languages (or a subset of the power set of the natural numbers) is "closed under many-one reducibility" if there exists no reduction from a language in C to a language outside C. If a class is closed under many-one reducibility, then many-one reduction can be used to show that a problem is in C by reducing a problem in C to it. Many-one reductions are valuable because most well-studied complexity classes are closed under some type of many-one reducibility, including P, NP, L, NL, co-NP, PSPACE, EXP, and many others. These classes