Project Euler #1 as an Ethereum contract (written in Solidity)

project-euler-001.sol

pragma solidity ^0.4.0;

contract ProjectEuler001 {
    // For the low price of 1000 wei, you can solve Project Euler Problem 1!
    uint price = 1000;

    event Solved(uint n, uint result);

    // Returns the n-th triangular number 1 + 2 + ... + n
    function t(uint n) private pure returns (uint) {
        return n * (n + 1) / 2;
    }

    // Solves Project Euler Problem 1 for the given input parameter (the input
    // for the actual problem should be 1000)
    function Solve(uint n) public payable {
        // This is not a charity :)
        require(msg.value >= price);

        // Subtract 1 since we only sum values strictly less than n
        uint m = n - 1;

        // Using inclusion-exclusion, the desired sum is
        // (3 + 6 + ...) + (5 + 10 + ...) - (15 + 30 + ...)
        // = 3 * (1 + 2 + ...) + 5 * (1 + 2 + ...) - 15 * (1 + 2 + ...)
        emit Solved(n, 3 * t(m / 3) + 5 * t(m / 5) - 15 * t(m / 15));
    }
}

This contract is deployed at address 0x5dD7BAFD0e478e73721D9AE9C6D9725a042F7dae