Lucas

Returns the desired Lucas number. Similar to the Fibonacci numbers, each Lucas number is defined to be the sum of its two immediate previous terms. The first two Lucas numbers are $L subscript 0 equals 2$ , $L subscript 1 equals 1$ and for $n$ greater than 2, $L subscript n equals L subscript n minus 1 end subscript plus L subscript n minus 2 end subscript$ .