To simplify the expression, we can rewrite it as:

a = 2008^(1.9.4.6).(1.9.4.7)...(1.9.9.9)
= 2008^(1.9.4.6 + 1.9.4.7 + ... + 1.9.9.9)

Now, let's break down the exponents:

1.9.4.6 + 1.9.4.7 + ... + 1.9.9.9

We can see that the exponents follow a pattern. The first exponent is 1.9.4.6, and each subsequent exponent increases by 1.9. For example, the second exponent is 1.9.4.7, the third exponent is 1.9.4.8, and so on.

To find the sum of these exponents, we can use the formula for the sum of an arithmetic series:

S = (n/2)(first term + last term)

In this case, the first term is 1.9.4.6 and the last term is 1.9.9.9. We need to find the number of terms, which is the difference between the last term and the first term, plus 1:

Number of terms = (1.9.9.9 - 1.9.4.6) + 1

Now, let's calculate the number of terms:

Number of terms = (1.9.9.9 - 1.9.4.6) + 1
= (1999 - 1946) + 1
= 54

Using this information, we can find the sum of the exponents:

S = (n/2)(first term + last term)
= (54/2)(1.9.4.6 + 1.9.9.9)

Now, let's calculate the sum of the exponents:

S = (54/2)(1.9.4.6 + 1.9.9.9)
= 27(1.9.4.6 + 1.9.9.9)

Finally, we can substitute this value back into the original expression:

a = 2008^S
= 2008^(27(1.9.4.6 + 1.9.9.9))

Unfortunately, without knowing the exact values of the exponents, we cannot simplify this expression further.