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A213506
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Number of nonisomorphic 2-generator p-groups of class at most 2 and order p^n.
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1
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1, 1, 2, 4, 6, 8, 13, 17, 23, 31, 40, 50, 65, 79, 97, 119, 143, 169, 203, 237, 277, 323, 373, 427, 492, 558, 633, 717, 807, 903, 1014, 1128, 1254, 1392, 1539, 1695, 1870, 2050, 2246, 2458, 2682, 2918, 3178, 3446, 3734, 4042, 4366, 4706, 5075, 5455, 5860
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OFFSET
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0,3
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LINKS
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FORMULA
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a(n) = sum_{r+s+t=n, r >= s >= t >= 0}( (t+1)+(1/2)*min{t,r-s}*(2t+1-min{t,r-s} ).
G.f.: 1/((1-x)*(1-x^2)*(1-x^3)^2*(1-x^4)). - Tani Akinari, Jun 28 2014
a(n) = floor( (n^4+26*n^3+234*n^2+(909-64*(n mod 3))*n+1701)/1728+(n+1)*(-1)^n/64 ). [Tani Akinari, Jun 28 2014 - see PARI code]
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MAPLE
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a := 0 ;
for t from 0 to n do
for s from t to n-t do
r := n-s-t ;
if r >= s then
m := min(t, r-s) ;
a := a+t+1+m*(2*t+1-m)/2 ;
end if;
end do:
end do:
return a;
end proc:
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MATHEMATICA
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CoefficientList[Series[1/((1 - x)*(1 - x^2)*(1 - x^3)^2*(1 - x^4)), {x, 0, 50}], x] (* Wesley Ivan Hurt, Jun 28 2014 *)
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PROG
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(PARI) a(n)=floor((n^4+26*n^3+234*n^2+(909-64*(n%3))*n+1701)/1728+(n+1)*(-1)^n/64) \\ Tani Akinari, Jun 28 2014
(PARI) Vec( 1/((1-x)*(1-x^2)*(1-x^3)^2*(1-x^4)) + O(x^100)) \\ Michel Marcus, Jun 28 2014
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CROSSREFS
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KEYWORD
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nonn,easy
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AUTHOR
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STATUS
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approved
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