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A082123 Smallest difference > 1 between d and p/d for any divisor d of the partial product p of the sequence, starting with 17. 2
17, 16, 26, 36, 76, 94, 432, 37220, 996768, 158267352, 973348166592, 8429202561226344, 419324164827901536306744, 339991740461303603766175692597227316, 12025891484499365294341150949542442100059557280661504 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

Table of n, a(n) for n=1..15.

MAPLE

branch:= proc(m, dm, bestyet)

    local t, x, nby, r;

    nby:= bestyet;

    for t from F[m][2] by -1 to 0 do

      x:= dm*F[m][1]^t;

      if x >= nby then next

      elif x >= c then nby:= x

      elif (x*R[m] < c) or (m=nF) then break

      else nby:= branch(m+1, x, nby);

      fi

    od;

    nby

end proc:

P:= 17: A[1]:= 17:

for n from 2 to 15 do

  c:= ceil(1/2+1/2*sqrt(5+4*P));

  while not type(c, integer) do Digits:= 2*Digits; c:= eval(c) od:

  F:= ifactors(P)[2]; nF:= nops(F);

  F:= sort(F, (s, t)->s[1]>t[1]);

  R:= [seq(mul(F[i][1]^F[i][2], i=j+1..nF), j=1..nF)];

  d:= branch(1, 1, P);

  A[n]:= d - P/d;

  P:= P*A[n]

od:

seq(A[n], n=1..15); # Robert Israel, May 20 2015

PROG

(PARI) p=17; print1(p, ", "); for(n=1, 13, r=floor(sqrt(p)); d1=1; d2=1; nE=omega(p); P=factor(p); E=P[, 2]; P=P[, 1]; forvec(v=vector(nE, i, [0, E[i]]), x=prod(k=1, nE, P[k]^v[k]); if(x<=r && x>=d2, d1=d2; d2=x, if(x<=d2 && x>=d1, d1=x))); difer=p/d2-d2; if(difer<=1, difer=p/d1-d1); print1(difer", "); p*=difer)

CROSSREFS

Cf. A082120, A003681 (starts with 2, 3), A082124.

Sequence in context: A022973 A023459 A004458 * A273973 A172091 A291370

Adjacent sequences:  A082120 A082121 A082122 * A082124 A082125 A082126

KEYWORD

nonn,hard,more

AUTHOR

Ralf Stephan, Apr 04 2003

EXTENSIONS

a(12) from Herman Jamke (hermanjamke(AT)fastmail.fm), Nov 02 2006

a(13) from Herman Jamke (hermanjamke(AT)fastmail.fm), Nov 14 2006

a(14) and a(15) from Robert Israel, May 20 2015

STATUS

approved

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Last modified August 22 00:43 EDT 2019. Contains 326169 sequences. (Running on oeis4.)