ographical , ▁or ▁historical ▁errors , ▁or ▁making ▁substantial ▁suggestions ▁for ▁his ▁publications . ▁The ▁M IT ▁Technology ▁Review ▁describes ▁the ▁checks ▁as ▁" am ong ▁comput erd om ' s ▁most ▁pri zed ▁tro ph ies ". ▁ ▁History ▁ ▁Initial ly , ▁Kn uth ▁sent ▁real , ▁negoti able ▁checks ▁to ▁recip ients . ▁He ▁stopped ▁doing ▁so ▁in ▁October ▁ 2 0 0 8 ▁because ▁of ▁problems ▁with ▁check ▁fra ud . ▁As ▁a ▁replacement , ▁he ▁started ▁his ▁own ▁" B ank ▁of ▁San ▁Ser rif fe ", ▁in ▁the ▁fict ional ▁nation ▁of ▁San ▁Ser rif fe , ▁which ▁keeps ▁an ▁account ▁for ▁everyone ▁who ▁found ▁an ▁error ▁since ▁ 2 0 0 6 . ▁Kn uth ▁now ▁sends ▁out ▁" hex ade c imal ▁certific ates " ▁instead ▁of ▁negoti able ▁checks . ▁ ▁, ▁Kn uth ▁reported ▁having ▁written ▁more ▁than ▁ 2 , 0 0 0 ▁checks , ▁with ▁an ▁average ▁value ▁exceed ing ▁$ 8 ▁per ▁check . ▁, ▁the ▁total ▁value ▁of ▁the ▁checks ▁signed ▁by ▁Kn uth ▁was ▁over ▁$ 2 0 , 0 0 0 . ▁Very ▁few ▁of ▁these ▁checks ▁were ▁actually ▁c ashed , ▁even ▁the ▁largest ▁ones . ▁More ▁often ▁they ▁have ▁been ▁fr amed ▁and ▁kept ▁as ▁" b rag ging ▁rights ". ▁ ▁Am ount ▁In ▁the ▁pre face ▁of ▁each ▁of ▁his ▁books ▁and ▁on ▁his ▁website , ▁Kn uth ▁offers ▁a ▁reward ▁of ▁$ 2 . 5 6 ▁( US D ) ▁to ▁the