Project Euler #2: Even Fibonacci numbers
I see a number of things that may help you improve your code.

Make it clear when loops end

The code currently uses while (true) but it doesn't really loop infinitely.  It's better, generally, to have the loop exit condition explicitly stated.  For example we could rewrite your main loop as:

unsigned long total = 0;
while (front &lt;= maxNumber) {
    total += front;
    unsigned long frontPrime = front;
    front = (3 * front) + (2 * back);
    back = (2 * frontPrime) + back;
}


Analyze the mathematics

Your mathematical analysis is correct as far as it goes, but there's a bit easier way to do it.  A bit more algebra will convince you that the series looks like this:

$$a_0 = 0$$
$$a_1 = 2$$
$$a_n = 4a_{n-1} + a_{n-2}$$

Derivation of this sequence

The Fibonacci numbers are defined as:
$$F_0 = 0$$
$$F_1 = 1$$
$$F_n = F_{n-1} + F_{n-2}$$
As you've already noted, the sum of two odd number is even, and the sum of an odd and even number is odd. For the Fibonacci sequence, this means that the sequence of \$O\$ (odd) and \$E\$ (even) looks like this:

$$E, O, O, E, O, O, E, ...$$
In other words, every third term is even.  This gets us to the fact that \$a_n = F_{3n}\$. From this observation, we have this:

$$a_n = F_{3n}$$
$$a_{n-1} = F_{3(n-1)} = F_{3n-3}$$
$$a_{n-2} = F_{3(n-2)} = F_{3n-6}$$
$$a_n = F_{3n-1} + F_{3n-2}$$
Using the definition of the Fibonacci sequence, and expanding the terms on the right, this becomes:
$$a_n = F_{3n-2} + F_{3n-3} + F_{3n-3} + F_{3n-4}$$
Substituting \$a\$ terms for \$F\$ terms, we have:
$$a_n = F_{3n-2} + 2a_{n-1} + F_{3n-4}$$
Expanding the \$F\$ terms on the right again using the definition of the Fibonacci sequence yields:
$$a_n = F_{3n-3} + F_{3n-4} + 2a_{n-1} + F_{3n-5} + F_{3n-6}$$
Again, substitute \$a\$ terms for \$F\$ terms:
$$a_n = a_{n-1} + F_{3n-4} + 2a_{n-1} + F_{3n-5} + a_{n-2}$$
Doing one last substitution and rearranging terms, we get:
$$a_n = 4a_{n-1} + a_{n-2}$$

Code implementation

That means that the outer loop could instead be written like this:

unsigned long back = 0;
unsigned long front = 2;
unsigned long current=0;
unsigned long total = front;
unsigned long maxNumber;
std::cin &gt;&gt; maxNumber;
while (front &lt;= maxNumber) {
    total += current;
    current = 4*front + back;
    back = front;
    front = current;
}
std::cout &lt;&lt; total &lt;&lt; '\n';


In this scheme \$a_{n-2}\$ is named back and \$a_{n-1}\$ is named front.  Naturally, \$a_n\$ is named current.

Don't recompute values

The sequence is the same every time, so it really doesn't make much sense to recompute values every time.  Better would be to compute them once and save them in a std::vector or std::array. Even better is to compute the values once at compile time.  Given the input range, there are less than 30 terms, which is a very modest storage requirement.

Omit return 0

When a C or C++ program reaches the end of main the compiler will automatically generate code to return 0, so there is no need to put return 0; explicitly at the end of main.  

Note: when I make this suggestion, it's almost invariably followed by one of two kinds of comments:  "I didn't know that." or "That's bad advice!"  My rationale is that it's safe and useful to rely on compiler behavior explicitly supported by the standard.  For C, since C99; see ISO/IEC 9899:1999 section 5.1.2.2.3:


  [...] a return from the initial call to the main function is equivalent to calling the exit function with the value returned by the main function as its argument; reaching the } that terminates the main function returns a value of 0.


For C++, since the first standard in 1998; see ISO/IEC 14882:1998 section 3.6.1:


  If control reaches the end of main without encountering a return statement, the effect is that of executing return 0;


All versions of both standards since then (C99 and C++98) have maintained the same idea.  We rely on automatically generated member functions in C++, and few people write explicit return; statements at the end of a void function.  Reasons against omitting seem to boil down to "it looks weird".  If, like me, you're curious about the rationale for the change to the C standard read this question.  Also note that in the early 1990s this was considered "sloppy practice" because it was undefined behavior (although widely supported) at the time.  

So I advocate omitting it; others disagree (often vehemently!)  In any case, if you encounter code that omits it, you'll know that it's explicitly supported by the standard and you'll know what it means.