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%I #13 Mar 02 2024 13:54:06
%S 0,1,7,1782,9211,13213,212722,252333,371911,766459,3829753,5743366,
%T 6534297,6566671,13336785,15347493,15973168,17831596,17965381,
%U 19567813,27335662,33154947,37494513,47539261,51817693,78335613,93657421
%N Heptagonal numbers for which the product of the digits is also a heptagonal number.
%H Harvey P. Dale, <a href="/A117661/b117661.txt">Table of n, a(n) for n = 1..130</a>
%e 766459 is in the sequence because it is a heptagonal number and the product of its digits (45360) is also a heptagonal number
%p a:=proc(n) local hn,hnn: hn:=convert(n*(5*n-3)/2,base,10): hnn:=product(hn[j],j=1..nops(hn)): if type((3+sqrt(9+40*hnn))/10,integer)=true then n*(5*n-3)/2 else fi end: seq(a(n),n=0..10000); # _Emeric Deutsch_, Apr 16 2006
%t Join[{0},Select[LinearRecurrence[{3,-3,1},{0,1,7},7000],IntegerQ[(3+ Sqrt[ 40*Times@@IntegerDigits[#]+9])/10]&]] (* _Harvey P. Dale_, Jun 17 2015 *)
%Y Cf. A000566.
%K nonn,base
%O 1,3
%A Luc Stevens (lms022(AT)yahoo.com), Apr 11 2006
%E More terms from _Emeric Deutsch_, Apr 16 2006
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