一も二も無く[ichimonimonaku] unhesitatingly
一言二言[hitokotofutakoto] a word or two
一石二鳥[issekinichou] killing two birds with one stone
一二に及ばず[ichininioyobazu] right away
一姫二太郎[ichihimenitarou] It's good to have a girl first and then a boy

うり二つ[urifutatsu] as alike as two melons (two peas in a pod)
黄道十二宮[koudoujuunikyuu] twelve signs of the zodiac
二度[futatabi] again
最小二乗法[saishounijouhou] least-squares method
三十二分音符[sanjuunibuonpu] 32nd note

乗冪[joubeki] power
冪乗[bekijou] a power
冪零[bekirei] nilpotent
冪乗演算子[bekijouenzanshi] exponential operator (**)

冪集合[bekishuugou] power set
冪級数[bekikyuusuu] power series
冪々[bekibeki] billowing (clouds, dust)
２の冪[ninobeki] power of two