Edith Cohen Nick G. Duffield Haim Kaplan Carsten Lund Mikkel Thorup Stream sampling for variance-optimal estimation of subset sums. 1255-1264 2009 SODA http://doi.acm.org/10.1145/1496770.1496906 conf/soda/2009 db/conf/soda/soda2009.html#CohenDKLT09