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%I #11 Aug 16 2015 12:04:01
%S 18,1877922,194706720450,20187582187830642,2093088896203949915058,
%T 217015642916030352905224578,22500615886726770153715544792802,
%U 2332908856150589340161504762302084050,241880656000904788079898366611289133690962
%N Positive heptagonal numbers (A000566) that are squares (A000290) divided by 2.
%C Intersection of A000566 and A001105 (even squares divided by 2). - _Michel Marcus_, Jun 20 2015
%H Colin Barker, <a href="/A259164/b259164.txt">Table of n, a(n) for n = 1..200</a>
%H <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (103683,-103683,1).
%F G.f.: -18*x*(x^2+646*x+1) / ((x-1)*(x^2-103682*x+1)).
%e 18 is in the sequence because 18 is the 3rd heptagonal number, and 2*18 is the 6th square.
%t LinearRecurrence[{103683, -103683, 1}, {18, 1877922, 194706720450}, 20] (* _Vincenzo Librandi_, Jun 20 2015 *)
%o (PARI) Vec(-18*x*(x^2+646*x+1)/((x-1)*(x^2-103682*x+1)) + O(x^20))
%Y Cf. A000290, A000566, A001105, A259156-A259163, A259165-A259167.
%K nonn,easy
%O 1,1
%A _Colin Barker_, Jun 19 2015