Question

The infinite sequence $T=\{t_0,t_1,t_2,\ldots\}$ is defined as $t_0=0,$ $t_1=1,$ and $t_n=t_{n-2}+t_{n-1}$ for all integers $n>1.$ If $a,$ $b,$ $c$ are fixed non-negative integers such that\begin{align*} a&\equiv 5\pmod {16}\\ b&\equiv 10\pmod {16}\\ c&\equiv 15\pmod {16}, \end{align*}then what is the remainder when $t_a+t_b+t_c$ is divided by $7?$ You can use a LaTeX renderer to see what this says.

Answer 1

$t_n$ are the Fibonacci numbers,

0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, ...

Taken modulo 7, we get the sequence

0, 1, 1, 2, 3, 5, 1, 6, 0, 6, 6, 5, 4, 2, 6, 1, ...

that repeats with period 16 (this is known as the 7th Pisano period).

This is to say,

$t_a\equiv t_{16n+5}\equiv t_5\equiv5\pmod7$

$t_b\equiv t_{16n+10}\equiv t_{10}\equiv6\pmod7$

$t_c\equiv t_{16n+15}\equiv t_{15}\equiv1\pmod7$

so that

$(t_a+t_b+t_c)\equiv(5+6+1)\equiv\boxed5\pmod7$