Use of hydrolyzed polymers of N-vinylcarboxamides as a means of increasing the dry strength of paper, board and cardboard

Proposed is the use, as agents for increasing the dry strength of paper, card and cardboard, of partially hydrolysed polymers of N-vinyl carboxylic acid amides obtained by the homopolymerization of N-vinyl carboxylic acid amides of the formula (I) in which R and R are H or C1-C6 alkyl and by splitting off 0.5 to 95 mole % of the groups of the formula (II) in which R is H or C1-C6 alkyl.