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A117650
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Heptagonal numbers for which the sum of the digits is also a heptagonal number.
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1
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0, 1, 7, 34, 189, 403, 783, 1782, 2673, 3186, 3744, 4347, 6426, 7209, 8037, 8910, 10791, 11122, 12852, 13950, 15093, 16281, 17514, 21022, 21483, 24354, 27405, 30636, 32319, 34047, 35820, 39501, 41409, 43362, 45360, 47403, 51624, 53802, 56025
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OFFSET
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0,3
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LINKS
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EXAMPLE
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196981 is in the sequence because it is a heptagonal number and the sum of its digits (34) is also a heptagonal number.
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MAPLE
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N:= 10000: # to search the first N heptagonal numbers
sd:= n -> convert(convert(n, base, 10), `+`):
hept:= x -> type((3+sqrt(9+40*x))/10, integer) or x = 0:
select(hept @ sd, [seq(n*(5*n-3)/2, n=0..N)]);
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PROG
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(PARI) isok(n) = ispolygonal(n, 7) && ispolygonal(sumdigits(n), 7);
for(n=0, 1e5, if(isok(n), print1(n, ", "))) \\ Altug Alkan, Dec 08 2015
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CROSSREFS
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KEYWORD
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nonn,base
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AUTHOR
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Luc Stevens (lms022(AT)yahoo.com), Apr 11 2006
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EXTENSIONS
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STATUS
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approved
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