Cauchy (1821) insisted on strict tests of convergence; he showed that if two series are convergent their product is not necessarily so, and with him begins the discovery of effective criteria.
Cauchy (1821) isihušul fal iruman oɣadnen ikanen ; saknen as afal əsin nilkaman aqalan aykan anmiƭaf nasan winan eqel ayfan, əd dər diris isinta ihanayan ən talɣiwen aśohatnen.