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 A130680 Numbers n such that n = (a_1 + a_2 + ... + a_p)*(a_1^3 + a_2^3 + ... + a_p^3), where n has the decimal expansion a_1a_2...a_p. 9
 1, 1215, 3700, 11680, 13608, 87949 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS This sequence is finite and all the terms are listed. Proof: Let a_1a_2...a_p be the decimal expansion of n. Then p <= log_10(n)+1. Furthermore we have a_i <= 9, therefore (a_1 + a_2 + ... + a_p) <= 9*(log_10(n)+1) and (a_1^3 + a_2^3 + ... + a_p^3) <= 9^3*(log_10(n)+1). On the other hand, for all n > 300000 we have 9^4*(log_10(n)+1)^2 < n. A computer search confirms that we indeed have found all terms. LINKS Table of n, a(n) for n=1..6. EXAMPLE 87949 = (8+7+9+4+9)*(8^3+7^3+9^3+4^3+9^3). MATHEMATICA For[n = 1, n < 1000000, n++, b = IntegerDigits[n]; If[Sum[b[[i]], {i, 1, Length[b]}] * Sum[b[[i]]^3, {i, 1, Length[b]}] == n, Print[n]]] ffQ[n_]:=Module[{c=IntegerDigits[n]}, Total[c]Total[c^3]==n]; Select[ Range[ 90000], ffQ] (* Harvey P. Dale, Oct 18 2013 *) CROSSREFS Cf. A115518. Sequence in context: A105311 A252125 A251815 * A346207 A068783 A331625 Adjacent sequences: A130677 A130678 A130679 * A130681 A130682 A130683 KEYWORD base,fini,full,nonn AUTHOR Yalcin Aktar, Jun 29 2007 EXTENSIONS Edited by Stefan Steinerberger, Jul 13 2007 STATUS approved

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Last modified June 17 06:25 EDT 2024. Contains 373432 sequences. (Running on oeis4.)