题目

How fit are your teeth? Are you lazy about brushing them? Never fear: An inventor is on the case. An electric toothbrush senses how long and how well you brush, and it lets you track your performance on your phone. The Kolibree toothbrush was exhibited at the International Consumer Electronics Show in Las Vegas this week. It senses how it is moved and can send the information to an Android phone or iPhone via a Bluetooth wireless connection. The toothbrush will be able to teach you to brush right (don’t forget the insides of the teeth!) and make sure you’re brushing long enough. 设函数f（x）（x∈N）表示x除以2的余数，函数g（x）（x∈N）表示x除以3的余数，则对任意的x∈N，给出以下式子：①f（x）≠g（x）；②g（2x）=2g（x）；③f（2x）=0；④f（x）+f（x+3）=1．其中正确的式子编号是    ．（写出所有符合要求的式子编号）