uz zy ▁sets . ▁ ▁Work ▁on ▁type - 2 ▁f uz zy ▁sets ▁l angu ished ▁during ▁the ▁ 1 9 8 0 s ▁and ▁early - to - mid ▁ 1 9 9 0 s , ▁although ▁a ▁small ▁number ▁of ▁articles ▁were ▁published ▁about ▁them . ▁People ▁were ▁still ▁trying ▁to ▁figure ▁out ▁what ▁to ▁do ▁with ▁type - 1 ▁f uz zy ▁sets , ▁so ▁even ▁though ▁Z ade h ▁proposed ▁type - 2 ▁f uz zy ▁sets ▁in ▁ 1 9 7 6 , ▁the ▁time ▁was ▁not ▁right ▁for ▁research ers ▁to ▁drop ▁what ▁they ▁were ▁doing ▁with ▁type - 1 ▁f uz zy ▁sets ▁to ▁focus ▁on ▁type - 2 ▁f uz zy ▁sets . ▁This ▁changed ▁in ▁the ▁latter ▁part ▁of ▁the ▁ 1 9 9 0 s ▁as ▁a ▁result ▁of ▁Prof . ▁Jerry ▁M end el ▁and ▁his ▁student ' s ▁works ▁on ▁type - 2 ▁f uz zy ▁sets ▁and ▁systems . ▁Since ▁then , ▁more ▁and ▁more ▁research ers ▁around ▁the ▁world ▁are ▁writing ▁articles ▁about ▁type - 2 ▁f uz zy ▁sets ▁and ▁systems . ▁ ▁Inter val ▁Type - 2 ▁F uz zy ▁S ets ▁Inter val ▁type - 2 ▁f uz zy ▁sets ▁have ▁received ▁the ▁most ▁attention ▁because ▁the ▁mathematics ▁that ▁is ▁needed ▁for ▁such ▁sets — prim arily ▁Inter val ▁arithmetic — is ▁much ▁simpler ▁than ▁the ▁mathematics ▁that ▁is ▁needed ▁for ▁general ▁type - 2 ▁f uz zy ▁sets . ▁So , ▁the ▁literature ▁about