My solutions to different of Project Euler's mathematical problems:

Euler001: Multiples of 3 and 5

• Add the numbers which have a zero modulo 3 or 5.

Euler002: Sum of even Fibonacci numbers

• Populate an array with all fibonacci numbers below a certain threshold,
• sum up the even (=modulo 2 gives no remainder) numbers from the array of fibonacci numbers.
• Note: identifying fibonacci numbers with a recursion gets too slow for n > 40 therefore different approach (see code).

Euler003: Largest prime factor of the given number 600,851,475,143.

• This one realy gave me a hard time!
• Finding a solution which worked for 13,195 was quite easy:
  • populate an array with all the factors of the given number,
  • identify the prime numbers in the array with the factors; the last one is the largest prime factor.
• My problem: for numbers greater than 1,000,000,000 (a billion) it took ages; calculated ~200 days of run time for the given number!
• My next solution calculated the prime factors backwards, starting with the largest one to reduce the number of calculations.
  • Good idea, but identifying whether this (big) number is a prime number still took far too long.
• Finally came across the idea that I only need to calculate from or until the square root of the given number which is a comparable handy number; this works!

Euler004: Largest palindrome product.

• Calculate each product of two 3-digit numbers,
• check for the product, whether it is a palindrome or not,
• store the palindroms in an array,
• find the largest number (=palindrom) in the array.

Euler005: Smallest positive number that is evenly divisible by all of the numbers from 1 to 20.

• Kind of a brute force approach, but it does work
  • going up from 1, as the smallest number is to be found
    • Test whether x can be divided by 1, 2, 3, ... n
      • yes? done
        • no? try next number
  • do it again until (1 * 2 * 3 * ... * n)

Euler006: Sum square difference:

• This one was pretty easy:
  • Calculate the sum of squares for (up to) the given number
  • Calculate the square of sums for (up to) the given number
    • Subtract them.

Euler007: 10001st prime

• First approach: might take too long?!
  • until last index of array with prime numbers is <= 10.001 do
    • look whether nbr is a prime nbr
      • if so, put it in array with prime numbers at index i
        • add one to i
      • add one to nbr
    • end
• But works...

Euler008: Largest product in a series

• Put each digit of the 1000 digit large number into an element of an array
• For each element of the array
  • calculate the sum of itself with its next 3 (12) following digits and
    • put the sum into another array
• The biggest element (max) of the other array is the one