%I A046175
%S A046175 0,1,20,285,3976,55385,771420,10744501,149651600,2084377905,29031639076,
%T A046175 404358569165,5631988329240,78443478040201,1092576704233580,
%U A046175 15217630381229925,211954248632985376,2952141850480565345
%N A046175 Indices of triangular numbers which are also pentagonal.
%H A046175 Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/
               PentagonalTriangularNumber.html">Link to a section of The World of 
               Mathematics.</a>
%F A046175 a(n) = 14*a(n-1) - a(n-2) + 6; g.f.: (1+5*x)/((1-x)*(1-14*x+x^2)) - Warut 
               Roonguthai (warut822(AT)yahoo.com) Jan 05 2001
%F A046175 a(n+1)=7*a(n)+3+2*(12*a(n)^2+12*a(n)+1)^0.5 - Richard Choulet (richardchoulet(AT)yahoo.fr), 
               Sep 19 2007
%F A046175 a(n)=-(1/2)+(1/12)*sqrt(3)*{[7-4*sqrt(3)]^n-[7+4*sqrt(3)]^n}+(1/4)*{[7+4*sqrt(3)]^n+[7-4*sqrt(3)]^n 
               }, with n>=0 [From Paolo P. Lava (ppl(AT)spl.at), Nov 25 2008]
%F A046175 a(n+1)=15*a(n)-15*a(n-1)+ a(n-2) with a(1)=1, a(2)=20, a(3)=285. - Sture 
               Sjostedt (sture.sjostedt(AT)spray.se), May 29 2009
%Y A046175 Cf. A014979, A046174.
%Y A046175 Cf. A001834 A046174 A014979.
%Y A046175 Sequence in context: A017953 A016317 A021404 this_sequence A016314 A021164 
               A017918
%Y A046175 Adjacent sequences: A046172 A046173 A046174 this_sequence A046176 A046177 
               A046178
%K A046175 nonn,easy
%O A046175 0,3
%A A046175 Eric Weisstein (eric(AT)weisstein.com)