concentrate at some place
to gather thickly at a place. The moths concentrated at the window at night, attracted by the light. All the thirsty children concentrated at the water fountain.
concentrate someone or something at something
to cause people or things to gather at a place; to cause people or things to convene or converge at a place. You shouldn't concentrate all the guards at one entrance. The general concentrated all the big guns at the entrance to the valley.
concentrate something on someone or something
to focus something on someone or something; to center on someone or something. Let's try to concentrate our efforts on finishing this job today. She concentrated her attention on Lynn.
concentrate (up)on someone or something
to focus one's thinking on someone or something; to think intensely about someone or something. (Upon is formal and less commonly used than on.) Please concentrate upon Jeff. He is the one we should discuss. Try to concentrate on your work more.