A doughnut.

 It got tired to make blog of the mathematics.And because I like food I will write about it.

     Seven-Eleven will full-scale sale of donuts from next year.
(Seven-Eleven is a convenience store that represents Japan.)
Coffee that is sold in the store is a popular taste so good.
A Doughnut is good the coffee and very compatible.
I think that doughnut as sell as coffee.

 　　But I worried about that Mister Donut sales retreat.
 Seven-Eleven of the donut and Mister Donut it, very similar.

↑Left Seven-Eleven. Right Mister Donut.

 

　 Donut will be launched in the country of Seven-Eleven in 2015.
I will note how both the sales change how.