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A117051
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Enneagonal numbers whose sum of digits is also enneagonal.
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1
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0, 1, 9, 261, 969, 1350, 6666, 7944, 10071, 13299, 17466, 24486, 33369, 36159, 39804, 42846, 46806, 54375, 57921, 66309, 75264, 80484, 111696, 116754, 128544, 135339, 153825, 167316, 175056, 181374, 204369, 226950, 235950, 243276, 252591, 260169
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OFFSET
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1,3
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LINKS
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EXAMPLE
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1350 is in the sequence because (1) it is an enneagonal number and (2) the sum of its digits 1+3+5+0=9 is also an enneagonal number
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MAPLE
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enne:= proc(x) issqr(56*x+25) and sqrt(56*x+25) mod 7 = 2 end proc:
enne(0):= true:
select(t -> enne(convert(convert(t, base, 10), `+`)), [seq(n*(7*n-5)/2, n=0..1000)]); # Robert Israel, Sep 20 2023
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MATHEMATICA
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Join[{0}, Select[Table[(n(7n-5))/2, {n, 300}], IntegerQ[(5+Sqrt[25+ 56*Total[ IntegerDigits[#]]])/14]&]] (* Harvey P. Dale, Apr 28 2016 *)
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PROG
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(PARI) isok(n) = ispolygonal(n, 9) && ispolygonal(sumdigits(n), 9); \\ Michel Marcus, Feb 26 2014
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CROSSREFS
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KEYWORD
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base,nonn
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AUTHOR
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Luc Stevens (lms022(AT)yahoo.com), Apr 15 2006
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STATUS
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approved
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