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 A117661 Heptagonal numbers for which the product of the digits is also a heptagonal number. 1
 0, 1, 7, 1782, 9211, 13213, 212722, 252333, 371911, 766459, 3829753, 5743366, 6534297, 6566671, 13336785, 15347493, 15973168, 17831596, 17965381, 19567813, 27335662, 33154947, 37494513, 47539261, 51817693, 78335613, 93657421 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 LINKS Harvey P. Dale, Table of n, a(n) for n = 1..130 EXAMPLE 766459 is in the sequence because it is a heptagonal number and the product of its digits (45360) is also a heptagonal number MAPLE 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 MATHEMATICA 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 *) CROSSREFS Cf. A000566. Sequence in context: A203697 A001424 A055318 * A206459 A172937 A185407 Adjacent sequences: A117658 A117659 A117660 * A117662 A117663 A117664 KEYWORD nonn,base AUTHOR Luc Stevens (lms022(AT)yahoo.com), Apr 11 2006 EXTENSIONS More terms from Emeric Deutsch, Apr 16 2006 STATUS approved

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Last modified August 7 22:54 EDT 2024. Contains 375018 sequences. (Running on oeis4.)