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 A159000 Numbers n such that there exist two numbers a and b where n=a.b=phi(a)*sigma(b)("." means concatenation). 4
 3360, 19440, 35712, 55800, 120960, 395808, 451584, 548640, 628992, 695520, 763344, 3008768, 3749760, 5602320, 17557344, 46902240, 55031040, 119627904, 162496048, 193933440, 243855360, 249793920, 374473800, 377677440, 548402400 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS A159001(n) give the first part a of a(n). LINKS Giovanni Resta, Table of n, a(n) for n = 1..33 EXAMPLE 3008768 = phi(3008)*sigma(768) so 3008768 is in the sequence. MATHEMATICA ntnQ[n_]:=MemberQ[(EulerPhi[#[[1]]]DivisorSigma[1, #[[2]]])==n&/@ Table[FromDigits/@TakeDrop[IntegerDigits[n], i], {i, IntegerLength[ n]-1}], True]; Select[Range[55*10^7], ntnQ] (* The program uses the TakeDrop function from Mathematica version 10 *) (* The program takes a long time to run *) (* Harvey P. Dale, Jan 01 2016 *) PROG (PARI) isA159000(n)={my(m); for(i=1, #Str(n)-1, m=n%10^i; if(m, m=divrem(n, sigma(m)); if(m[2]==0&eulerphi(n\10^i)==m[1], return(i)))); 0} /* Charles R Greathouse IV, Apr 28 2010 */ CROSSREFS Cf. A159001, A253824, A260144. Sequence in context: A104938 A179704 A222154 * A032773 A323965 A223295 Adjacent sequences:  A158997 A158998 A158999 * A159001 A159002 A159003 KEYWORD nonn,base AUTHOR Farideh Firoozbakht, Jun 04 2009 EXTENSIONS Corrected (a(3), a(4), and a(11) missing), extended past a(12), and edited by Charles R Greathouse IV, Apr 28 2010 STATUS approved

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Last modified April 20 03:02 EDT 2021. Contains 343121 sequences. (Running on oeis4.)