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Tricoloured Coin Fountains

Problem 519 Published on Saturday, 6th June 2015, 07:00 pm; Solved by 263;
Difficulty rating: 50%

An arrangement of coins in one or more rows with the bottom row being a block without gaps and every coin in a higher row touching exactly two coins in the row below is called a fountain of coins. Let f(n) be the number of possible fountains with n coins. For 4 coins there are three possible arrangements:


Therefore f(4) = 3 while f(10) = 78.

Let T(n) be the number of all possible colourings with three colours for all f(n) different fountains with n coins, given the condition that no two touching coins have the same colour. Below you see the possible colourings for one of the three valid fountains for 4 coins:


You are given that T(4) = 48 and T(10) = 17760.

Find the last 9 digits of T(20000).