infinite matrix brain teaser

imvalue.py

def summation(n):
    return ((n**2 + n)/2) + 1

def imvalue(i, j):
    if ((j + i - 1) % 2 == 0):
        return summation(j + i - 1) - i
    else:
        return summation(j + i - 1) - j

#edges that are starts of streams
print(imvalue(1,3)) #4
print(imvalue(1,5)) #11
print(imvalue(4,1)) #7
print(imvalue(2,1)) #2

#edges that are not starts
print(imvalue(1,4)) #10
print(imvalue(1,6)) #21
print(imvalue(3,1)) #6
print(imvalue(6,1)) #16

#values in the middle of upstream
print(imvalue(2,3)) #9
print(imvalue(2,5)) #20
print(imvalue(4,3)) #18
print(imvalue(3,2)) #8

#values in the middle of downstream
print(imvalue(2,4)) #12
print(imvalue(2,6)) #23
print(imvalue(2,2)) #5
print(imvalue(6,2)) #27