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 A114617 Numbers k such that k and k+1 are both refactorable numbers. 15
 1, 8, 1520, 50624, 62000, 103040, 199808, 221840, 269360, 463760, 690560, 848240, 986048, 1252160, 1418480, 2169728, 2692880, 2792240, 3448448, 3721040, 3932288, 5574320, 5716880, 6066368, 6890624, 6922160, 8485568 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS It is not possible to have three consecutive refactorable numbers (see the link). The sequence is best viewed in base 12, with X for 10 and E for 11: 1, 8, X68, 25368, 2EX68, 4E768, 97768, X8468, 10EX68, 1X4468, 293768, 34XX68, 3E6768, 504768, 584X68, 887768, X9X468, E27X68, 11X3768, 12E5468, 1397768, 1X49X68, 1XE8468, 2046768, 2383768, 2399X68, 2X12768. After the first two terms all terms are 68, 368, 468, 668, 768, X68 mod 1000 (base 12). - Walter Kehowski, Jun 19 2006 No successive refactorables seem to be of the form odd, odd+1. If such a pair exist, they must be very large. The first pair of successive refactorables not divisible by 3 is (5*19)^4-1, (5*19)^4. - Walter Kehowski, Jun 25 2006 Zelinsky (2002, Theorem 59, p. 15) proved that all the terms above 1 are even. - Amiram Eldar, Feb 20 2021 LINKS Jud McCranie, Table of n, a(n) for n = 1..94342 First 1000 terms by Donovan Johnson. Joshua Zelinsky, Tau Numbers: A Partial Proof of a Conjecture and Other Results, Journal of Integer Sequences, Vol. 5 (2002), Article 02.2.8. Eric Weisstein's World of Mathematics, Refactorable Number. FORMULA a(n) mod tau(a(n)) = 0 and (a(n)+1) mod tau(a(n)+1) = 0 where tau(n) is the number of divisors of n. - Walter Kehowski, Jun 19 2006 MAPLE with(numtheory); RFC:=[]: for w to 1 do for k from 1 to 12^6 do n:=144*k+(6*12+8); if andmap(z-> z mod tau(z) = 0, [n, n+1]) then RFC:=[op(RFC), n]; print(n); fi od od; # it is possible to remove the condition n = (6*12+8) mod 12^2 but you'll get the same sequence. - Walter Kehowski, Jun 19 2006 MATHEMATICA Select[Join[{1, 8}, 144*Range[10^5] + 80], Mod[#, DivisorSigma[0, #]] == 0 && Mod[#+1, DivisorSigma[0, #+1]] == 0 & ](* Jean-François Alcover, Oct 25 2012, after Walter Kehowski *) PROG (PARI) isok(n) = !(n % numdiv(n)) && !((n+1) % numdiv(n+1)); \\ Michel Marcus, Dec 21 2018 (GAP) Filtered([1..10^6], n->n mod Tau(n)=0 and (n+1) mod Tau(n+1)=0 ); # Muniru A Asiru, Dec 21 2018 CROSSREFS Cf. A033950, A036898. Sequence in context: A252763 A096970 A248386 * A162014 A300545 A300972 Adjacent sequences: A114614 A114615 A114616 * A114618 A114619 A114620 KEYWORD nonn AUTHOR Eric W. Weisstein, Dec 16 2005 STATUS approved

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Last modified August 10 22:52 EDT 2024. Contains 375059 sequences. (Running on oeis4.)