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 A254317 a(n) is the least number k such that the number of distinct digits in the prime factorization of k is n (counting terms of the form p^1 as p). 2
 1, 6, 26, 102, 510, 3210, 22890, 153690, 1507290, 15618090 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Write k as product of primes raised to powers; then a(n) is the least number k such that the total number of distinct digits in the product representation of k (number of distinct digits in all the primes and number of distinct digits in all the exponents that are greater than 1) is equal to n. The first term a(1)= 1 by convention. The sequence is complete. Property: all exponents are equal to 1 (see the examples below). LINKS EXAMPLE a(1)  =        1; a(2)  =        6 = 2*3            and A254315(6)        =  2; a(3)  =       26 = 2*13           and A254315(26)       =  3; a(4)  =      102 = 2*3*17         and A254315(102)      =  4; a(5)  =      510 = 2*3*5*17       and A254315(510)      =  5; a(6)  =     3210 = 2*3*5*107      and A254315(3210)     =  6; a(7)  =    22890 = 2*3*5*7*109    and A254315(22890)    =  7; a(8)  =   153690 = 2*3*5*47*109   and A254315(153690)   =  8; a(9)  =  1507290 = 2*3*5*47*1069  and A254315(1507290)  =  9; a(10) = 15618090 = 2*3*5*487*1069 and A254315(15618090) = 10. MAPLE with(ListTools): for n from 2 to 10 do:   ii:=0:   for k from 2 to 10^9 while(ii=0)do:     n0:=length(k):lst:={}:x0:=ifactors(k):     y:=Flatten(x0):z:=convert(y, set):     z1:=z minus {1}:nn0:=nops(z1):      for m from 1 to nn0 do :       t1:=convert(z1[m], base, 10):z2:=convert(t1, set):       lst:=lst union z2:      od:      nn1:=nops(lst):      if nn1=n then ii:=1:printf ( "%d %d \n", n, k):      else      fi:   od : od: MATHEMATICA f[n_] := Block[{pf = FactorInteger@ n, i}, Length@ DeleteDuplicates@ Flatten@ IntegerDigits@ Rest@ Flatten@ Reap@ Do[If[Last[pf[[i]]] == 1, Sow@ First@ pf[[i]], Sow@ FromDigits@ Flatten[IntegerDigits /@ pf[[i]]]], {i, Length@ pf}]]; b = -1; Flatten@ Last@ Reap@ Do[a = f[n]; If[a > b, Sow[n]; b = a], {n, 10^6}] (* Michael De Vlieger, Jan 29 2015 *) With[{s = Array[CountDistinct@ Flatten@ IntegerDigits[FactorInteger[#] /. {p_, e_} /; e == 1 :> {p}] &, 10^6]}, Map[FirstPosition[s, #][] &, Range@ Max@ s]] (* Michael De Vlieger, Nov 03 2017 *) PROG (PARI) a(n)=for(k=1, 10^5, s=[]; F=factor(k); for(i=1, #F[, 1], s=concat(s, digits(F[i, 1])); if(F[i, 2]>1, s=concat(s, digits(F[i, 2])))); if(#vecsort(s, , 8)==n, return(k))) print1(1, ", "); for(n=2, 7, print1(a(n), ", ")) \\ Derek Orr, Jan 30 2015 CROSSREFS Cf. A043537, A254315. Sequence in context: A143955 A256428 A196860 * A037545 A027996 A020989 Adjacent sequences:  A254314 A254315 A254316 * A254318 A254319 A254320 KEYWORD nonn,base,fini,full AUTHOR Michel Lagneau, Jan 28 2015 EXTENSIONS a(10) corrected by Giovanni Resta, Nov 03 2017 STATUS approved

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Last modified June 3 05:29 EDT 2020. Contains 334798 sequences. (Running on oeis4.)