# why $-$ sign in DTFT pair for constant

In discrete time Fourier transform, The DTFT of constant 1 is $$\sum_{l=-\infty}^{+\infty} \delta(\omega-2\pi l)$$. I have confusion that why there is $-$ sign, why it can't be $$\sum_{l=-\infty}^{+\infty} \delta(\omega+2\pi l)$$

It can also have a $+$ sign, there's no difference. Write down a part of the sum (around index $l=0$) and try to see that in both cases you're summing the same terms, just in a different order.
More formally, take one of the two sums, transform the index by changing its sign ($k=-l$) and see what you get.