Project Euler 2: Even Fibonacci Numbers

Project Euler 2 - Even Fibonacci Numbers


Each new term in the Fibonacci sequence is generated by adding the previous two terms. By starting with 1 and 2, the first 10 terms will be:

1, 2, 3, 5, 8, 13, 21, 34, 55, 89, …

By considering the terms in the Fibonacci sequence whose values do not exceed four million, find the sum of the even-valued terms.

Official Problem

Solution Approach

In this problem of even Fibonacci Numbers, we have to do a sum of even Fibonacci series. Let’s find a general approach first for solving this problem then let’s dive down to the more specific limiter like up to 4 million as asked in the question.

The problem can be approached in 2 ways – one can be a normal brute force technique which is generally used to find the Fibonacci series and secondly is the intelligent even Fibonacci sum by finding patterns and utilizing those patterns to perform the same.

Brute Force Approach

Let’s start with the brute force approach where we will set the first 2 numbers of the sequence as 1,2 as stated in the problem and find the third Fibonacci sequence by using the formula Fn = Fn-1 + Fn-2 so the next few terms in the series will be 3,5,8,13 and so on. We check all the numbers if they are even, which is generated through this formula, and calculate their sum.

Coming to the code part of this approach first declare initialize the sum as 0 then have another variable to store the nth value which is 4 million in our problem at hand. Set the nth term as an exit condition to the loop and iterate using the Fibonacci formula and generate the numbers. Also, keep checking if the number is even, and if it is so then add the number to the sum.

Optimized Approach

Now, moving to the more optimized answer to the problem we will first let’s see the series. We have

1, 2, 3, 5, 8, 13, 21, 34....

From the trend we can clearly see that every third character is an even number. We will use this information to solve the problem in hand

Now lets derive a relation of getting Fibonacci relation only between the even numbers.

If Fn is the nth Fibonacci number then Fn-1 and Fn-2 are the numbers before that. So we need to have a relation between Fn, Fn-3, and Fn-6 as even the Fibonacci number occurs as every 3rd Fibonacci number.

To derive the relation we already know that

  • Fn= Fn-1 + Fn-2
  • Fn = Fn-2 + Fn-3 + Fn-3 + Fn-4 (expanding Fn-1 = Fn-2 + Fn-3 and Fn-2 = Fn-3 + Fn-4)
  • Fn = Fn-3 + Fn-4 + 2Fn-3 + Fn-5 + Fn-6 (using the same logic we expand Fn-4 and Fn-2)
  • Fn = 4Fn-3 + Fn-6 (as Fn-4 + Fn-5 = Fn-3)

we can change it to EFn = 4EFn-1 + EFn-2 where EFn are even Fibonacci numbers.

Coming to the code part of this approach we initialize the sum variable to have a value of 0 and the limit variable to contain the value of 4 million. We already know the first 2 even numbers are 2 and 8 using the above formula we calculate the next terms by looping until the limit and we have to keep adding the numbers obtained from the formula to the sum variable. In the end, we will just print the sum variable’s value as the answer.

Solution Code

Brute Force Approach

Python Solution

Java Solution

Optimized Approach

Python Solution

Java Solution

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