%I A117709
%S A117709 0,1,5,651,1335,2262,3432,3577,6501,8400,8626,10542,10795,15862,18760,
%T A117709 21540,25285,28912,32340,32782,45850,50142,50692,55200,60501,72490,
%U A117709 91390,98945,104412,112477,127750,135751,152482,160230,170185,179401
%N A117709 Pentagonal numbers for which the sum of the digits is also a pentagonal 
               number.
%e A117709 651 is in the sequence because it is a pentagonal number and the sum 
               of its digits 6+5+1=12 is also a pentagonal number.
%p A117709 a:=proc(n) local P,s: P:=convert(n*(3*n-1)/2,base,10): s:=add(P[j],j=1..nops(P)): 
               if n=0 then 0 elif type((1+sqrt(1+24*s))/6,integer)=true then n*(3*n-1)/
               2 else fi end: seq(a(n),n=0..350); - Emeric Deutsch (deutsch(AT)duke.poly.edu), 
               Apr 15 2006
%Y A117709 Cf. A000326.
%Y A117709 Sequence in context: A060758 A068421 A142535 this_sequence A133750 A090947 
               A000367
%Y A117709 Adjacent sequences: A117706 A117707 A117708 this_sequence A117710 A117711 
               A117712
%K A117709 base,nonn
%O A117709 0,3
%A A117709 Luc Stevens (lms022(AT)yahoo.com), Apr 13 2006