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A117650 Heptagonal numbers for which the sum of the digits is also a heptagonal number. 1
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 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,2

LINKS

Robert Israel, Table of n, a(n) for n = 0..10000

EXAMPLE

196981 is in the sequence because it is a heptagonal number and the sum of its digits (34) is also a heptagonal number.

MAPLE

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):

select(hept @ sd, [seq(n*(5*n-3)/2, n=1..N)]);

# Robert Israel, Feb 14 2013

PROG

(PARI) isok(n) = ispolygonal(n, 7) && ispolygonal(sumdigits(n), 7);

for(n=1, 1e5, if(isok(n), print1(n, ", "))) \\ Altug Alkan, Dec 08 2015

CROSSREFS

Cf. A000566.

Sequence in context: A080048 A177140 A027233 * A273221 A144038 A027842

Adjacent sequences:  A117647 A117648 A117649 * A117651 A117652 A117653

KEYWORD

nonn,base

AUTHOR

Luc Stevens (lms022(AT)yahoo.com), Apr 11 2006

STATUS

approved

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Last modified November 18 17:45 EST 2019. Contains 329287 sequences. (Running on oeis4.)