cp's OEIS Frontend

A157017 Numbers n such that n! can be written as a product of distinct factors in the range from n+1 to 2n, inclusive.

%I A157017 #26 Apr 25 2019 03:33:21
%S A157017 3,6,8,11,14,15,18,21,22,25,28,29,32,35,39,40,43,44,47,48,51,52,55,56,
%T A157017 59,60,61,63,64,67,68,69,73,74,76,77,78,86,89,90,94,95,98,99,103,104,
%U A157017 107,116,117,122,123,124,125,126,127,131,145,146,149,158,159,179,183,187,188,189,191,194,203,207,215,218,219,221,222,223,224,229,230,233,238,239
%N A157017 Numbers n such that n! can be written as a product of distinct factors in the range from n+1 to 2n, inclusive.
%C A157017 Erdos remarks that this is a finite sequence. - _N. J. A. Sloane_, Feb 23 2009
%C A157017 Here is another way of displaying a representation of n!: Let cp(n) be the product of the composite numbers from n+1 to 2n, including the ends (A157625). For example, 40! = cp(40) / (46*70*77). Because the number of factors in the denominator is small relative to n, this simpler form gives us a fast method of finding representations of n!: find distinct factors of cp(n)/n! among the numbers n+1 to 2n. See A157229 for the number of representations of n! for the n in this sequence. - _T. D. Noe_, Feb 25 2009
%C A157017 Erdos et al. found this sequence and showed that 239 is the last term. Note that 239! has 94766 representations! Sequence A157229, which is also in the Erdos et al. paper, gives the number of representations for each n. Ray Chandler and I created an algorithm that verifies the numbers in both sequences. - _T. D. Noe_, Mar 01 2009
%D A157017 P. Erdos, R. K. Guy and J. L. Selfridge, Another property of 239 and some related questions, Proceedings of the Eleventh Manitoba Conference on Numerical Mathematics and Computing (Winnipeg, Man., 1981), Congr. Numer. 34 (1982), 243-257.
%H A157017 Ray Chandler, <a href="/A157017/a157017b.txt">Detailed examples for terms in A157017</a>
%H A157017 P. Erdos, <a href="http://www.renyi.hu/~p_erdos/1975-07.pdf">Consecutive integers</a>, Eureka, The Archimedeans' Journal, 38 (1975/76), 3-8.
%H A157017 P. Erdos, <a href="/A157017/a157017.pdf">Consecutive integers</a> (1975) [Cached copy]
%H A157017 P. Erdos, R. K. Guy and J. L. Selfridge, <a href="http://www.renyi.hu/~p_erdos/1982-01.pdf">Another property of 239 and some related questions</a> (1982)
%H A157017 T. D. Noe, <a href="/A157017/a157017.txt">Representations of n!</a>
%F A157017 A number n is in the sequence iff A000984(n)*A034386(n)/A034386(2n) is the product of distinct composite numbers in {n+1,...,2n}. - _M. F. Hasler_, Feb 10 2014
%e A157017 3! = 6. [Vaughan, quoted by Erdos]
%e A157017 6! = 8*9*10. [Erdos]
%e A157017 8! = 12*14*15*16. [Vaughan, quoted by Erdos]
%e A157017 11! = 15*16*18*20*21*22. [Vaughan, quoted by Erdos]
%e A157017 14! = 16*21*22*24*25*26*27*28. [Erdos]
%e A157017 15! = 16*18*20*21*22*25*26*27*28. [Vaughan, quoted by Erdos]
%e A157017 18! = 20*21*22*24*26*27*30*32*34*35*36 = cp(18) / (25*28*33).
%e A157017 18! = 20*21*24*25*26*27*28*32*33*34*36 = cp(18) / (22*30*35).
%e A157017 18! = 21*22*24*25*26*27*28*30*32*34*36 = cp(18) / (20*33*35).
%e A157017 21! = 24*25*27*28*32*33*34*35*36*38*39*40*42 = cp(21) / (22*26*30).
%e A157017 22! = 24*25*26*27*28*30*32*33*34*35*36*38*42*44 = cp(22) / (39*40).
%e A157017 25! = 26*27*30*32*33*34*35*36*38*40*44*45*46*48*49*50 = cp(25) / (28*39*42).
%e A157017 25! = 27*28*30*32*33*34*35*38*39*40*42*44*45*46*48*50 = cp(25) / (26*36*49).
%e A157017 28! = 30*32*33*36*38*39*40*42*45*46*48*49*50*51*52*54*55*56.
%e A157017 29! = 30*32*33*34*35*36*39*40*42*44*45*46*48*49*50*52*54*57*58.
%e A157017 29! = 30*32*33*35*36*38*39*40*42*44*45*46*48*49*50*51*52*54*58.
%e A157017 32! = 34*35*36*39*40*42*44*45*46*48*50*52*54*55*56*57*58*60*62*63*64
%e A157017 32! = 35*36*38*39*40*42*44*45*46*48*50*51*52*54*55*56*58*60*62*63*64
%e A157017 35! = 36*40*44*45*48*49*50*51*52*54*55*56*57*58*60*62*63*64*65*66*68*69*70
%e A157017 39! = 40*42*45*48*51*52*54*55*56*57*58*60*62*63*64*65*66*68*69*70*72*74*75*76*77*78
%e A157017 39! = 42*44*45*48*50*51*52*54*56*57*58*60*62*63*64*65*66*68*69*70*72*74*75*76*77*78
%e A157017 40! = 42*44*45*48*49*50*51*52*54*55*56*57*58*60*62*63*64*65*66*68*69*72*74*75*76*78*80. [Vaughan, quoted by Erdos]
%e A157017 43! = 44*48*49*50*52*54*57*58*60*62*63*64*65*66*68*69*70*72*74*75*76*77*78*80*81*82*84*85*86 (and 2 other ways)
%e A157017 44! = 45*46*48*49*50*51*52*54*55*56*57*60*62*64*65*66*70*72*74*76*77*78*80*81*82*84*85*86*87*88 (and 16 other ways)
%e A157017 See link for further example.
%o A157017 (PARI) is_A237594(n,m=2*n,p=binomial(2*n,n)/prod(k=primepi(n)+1,primepi(n*2),prime(k)))={forstep(f=m,n+1,-1, p%f==0 && (p==f || is_A237594(n,f-1,p/f)) && return(1))} \\ _M. F. Hasler_, Feb 10 2014
%Y A157017 Cf. A000142, A157625, A157229.
%K A157017 full,fini,nonn
%O A157017 1,1
%A A157017 _Jaume Oliver Lafont_, Feb 21 2009
%E A157017 More precise definition and term 18 from _R. J. Mathar_, Feb 21 2009 Terms 21 through 73 added by _Ray Chandler_ and _T. D. Noe_, further terms up to 158 by _T. D. Noe_, Feb 24 2009
%E A157017 Terms 159 to 239 added by _Ray Chandler_ and _T. D. Noe_, Mar 01 2009
%E A157017 Erroneous term 5 removed by Markus Koenig (markus(AT)stber-koenig.de), Mar 13 2010
%E A157017 Erroneous terms 75 and 88 removed by _T. D. Noe_, Apr 01 2010
%E A157017 Terms up to 160 double-checked by _M. F. Hasler_, Feb 10 2014