‌若使函数<img src="http://jgxm-question.oss-cn-beijing.aliyuncs.com/psst/files/cace0fc5032e40e08ed54e2e738f4990.png?x-oss-process=image/sharpen,200"/>在除原点的任何点处函数值增加最快的方向都是该点指向坐标原点的方向，则常数<img src="http://jgxm-question.oss-cn-beijing.aliyuncs.com/psst/files/f40a5980af0c40348edd52b169c00c71.png?x-oss-process=image/sharpen,200"/>必须满足