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Number Sequence Game

Published on Saturday, 23rd August 2014, 04:00 pm; Solved by 202;
Difficulty rating: 65%

Problem 477

The number sequence game starts with a sequence S of N numbers written on a line.

Two players alternate turns. At his turn, a player must select and remove either the first or the last number remaining in the sequence.

The player score is the sum of all the numbers he has taken. Each player attempts to maximize his own sum.

If N = 4 and S = {1, 2, 10, 3}, then each player maximizes his score as follows:
  • Player 1: removes the first number (1)
  • Player 2: removes the last number from the remaining sequence (3)
  • Player 1: removes the last number from the remaining sequence (10)
  • Player 2: removes the remaining number (2)

Player 1 score is 1 + 10 = 11.

Let F(N) be the score of player 1 if both players follow the optimal strategy for the sequence S = {s1, s2, ..., sN} defined as:

  • s1 = 0
  • si+1 = (si2 + 45) modulo 1 000 000 007

The sequence begins with S = {0, 45, 2070, 4284945, 753524550, 478107844, 894218625, ...}.

You are given F(2) = 45, F(4) = 4284990, F(100) = 26365463243, F(104) = 2495838522951.

Find F(108).