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This proof Q has two appealing features : it can be derived from P in a number of steps proportional to nc , where c is some constant ; and P can be verified by examining only three bits of Q picked at random . " Effective tolerance analysis is perhaps the most critical design step essential for reducing manufacturing costs while achieving optimal product performance goals . Due to the combination of small drainage area , stepped shallow gradient , large roughness elements , and cohesive sediments , headwater streams typically transport little sediment or coarse wood debris by fluvial processes . Then , in a key step vital to the virus 's production in human cells , a human protein called Sp1 attaches to the two ends of the viral DNA ; the end pieces of viral DNA are called the LTR regions , for " long terminal repeat . Cdk plus cyclin activates two successive steps essential to the cell cycle .