generating function:
a(x)=∑anxn
n=3∑∞(an−7an−1+16an−2−12an−3)xn=n=3∑∞n4nxn
n=3∑∞anxn=a(x)−a0−a1x−a2x2=a(x)+2−5x2
n=3∑∞7an−1xn=7xn=3∑∞an−1xn−1=7x(a(x)−a0−a1x)=7x(a(x)+2)
n=3∑∞16an−2xn=16x2n=3∑∞an−2xn−2=16x2(a(x)−a0)=16x2(a(x)+2)
n=3∑∞12an−3xn=12x3n=3∑∞an−3xn−3=12x3a(x)
n=3∑∞(an−7an−1+16an−2−12an−3)xn=
=a(x)+2−5x2−7x(a(x)+2)+16x2(a(x)+2)−12x3a(x)=
=a(x)(1−7x+16x2−12x3)−14x+25x2−12x3+2
n=3∑∞n4nxn=(1−4x)2x
a(x)=(1−7x+16x2−12x3)(1−4x)2x+1−7x+16x2−12x314x+25x2−12x3+2
a(x)=2x−16+(2x−1)2−1+4x−124+(4x−1)24+3x−1−27+2(2x−1)325+2(2x−1)2−55+4x−1647+(4x−1)2110+
+3x−1−729
a(x)=2(2x−1)337−2(2x−1)257+4x−1671+(4x−1)2114−3x−1756
a(x)=−2337n=0∑∞2nxn−671n=0∑∞4nxn+756n=0∑∞3nxn−257n=0∑∞(n+1n)2nxn+
+114n=0∑∞(n+1n)4nxn
an=−23372n−671⋅4n+756⋅3n−257(n+1)2n+114(n+1)4n
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