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EXAMPLE
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G.f.: A(x) = 1 + 4*x + 64*x^2 + 3072*x^3 + 466944*x^4 + 283115520*x^5 + ...
B(x) = 1 + 2*x + 10*x^2 + 188*x^3 + 16774*x^4 + 6745436*x^5 + ...
then a(n) is the coefficient of x^n in B(x)^(2^n):
B(x)^(2^0): [(1),2,10,188,16774,6745436,11466849412,...];
B(x)^(2^1): [1,(4),24,416,34400,13561728,22961051392,...];
B(x)^(2^2): [1,8,(64),1024,72704,27418624,46032420864,...];
B(x)^(2^3): [1,16,192,(3072),165888,56131584,92513894400,...];
B(x)^(2^4): [1,32,640,12288,(466944),118751232,186897137664,...];
B(x)^(2^5): [1,64,2304,65536,2129920,(283115520),382143037440,...];
B(x)^(2^6): [1,128,8704,425984,17956864,1140850688,(814634500096),...];
the terms along the diagonal (in parentheses) form this sequence. (End)
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