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A253882
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Number of 3-connected planar triangulations of the sphere with n vertices up to orientation preserving isomorphisms.
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2
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1, 1, 2, 6, 17, 73, 389, 2274, 14502, 97033, 672781, 4792530, 34911786, 259106122, 1954315346, 14949368524, 115784496932, 906736988527, 7171613842488, 57231089062625, 460428456484557, 3731572377382341, 30447133566946517, 249968326771680542, 2063931874299323140
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OFFSET
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4,3
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LINKS
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PROG
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(PARI) a(n)={if(n<3, 0, (2*binomial(4*(n-3)+1, n-3)/((n-2)*(3*n-7))
+ 3*sumdiv(n-2, d, if(d>=2, my(s=(n-2)/d); eulerphi(d)*binomial(4*s, s))/4)
+ if(n%2==1, my(s=(n-3)/2); 3*binomial(4*s, s)*(2*s+1)/(3*s+1))
+ if(n%3==1, my(s=(n-4)/3); 8*binomial(4*s, s)*(4*s+1)/(3*s+1))
+ if(n%3==0, my(s=(n-3)/3); 2*binomial(4*s, s)) )/(6*(n-2)))} \\ Andrew Howroyd, Mar 02 2021
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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Name clarified and terms a(24) and beyond from Andrew Howroyd, Mar 02 2021
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STATUS
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approved
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