To find the sum of the arithmetic series 1, 4, 7, ..., 100, 103, we can use the formula for the sum of an arithmetic series:

Sn = (n/2)(a1 + an)

where Sn is the sum of the series, n is the number of terms, a1 is the first term, and an is the last term.

In this case, a1 = 1, an = 103, and n can be calculated using the formula for the nth term of an arithmetic series:

an = a1 + (n-1)d

where d is the common difference.

In this case, d = 4 - 1 = 3.

Solving for n:

103 = 1 + (n-1)3
103 = 1 + 3n - 3
103 = 3n - 2
105 = 3n
n = 35

Now we can substitute the values into the sum formula:

Sn = (n/2)(a1 + an)
Sn = (35/2)(1 + 103)
Sn = (35/2)(104)
Sn = 35 * 52
Sn = 1820

Therefore, the sum of the arithmetic series 1, 4, 7, ..., 100, 103 is 1820.