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Numbers with multiplicative digital root value 4.
18

%I #40 Feb 13 2017 20:20:25

%S 4,14,22,27,39,41,72,89,93,98,114,122,127,139,141,172,189,193,198,212,

%T 217,221,249,266,271,277,294,319,333,338,346,364,379,383,391,397,411,

%U 429,436,463,492,626,634,643,662,677,712,721,727,739,767,772,776,793

%N Numbers with multiplicative digital root value 4.

%H Robert Israel, <a href="/A034051/b034051.txt">Table of n, a(n) for n = 1..5000</a>

%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/MultiplicativeDigitalRoot.html">Multiplicative Digital Root</a>

%H <a href="/index/Ar#10-automatic">Index entries for 10-automatic sequences</a>.

%p mdr:= proc(n) option remember;

%p if n < 10 then return(n) fi;

%p procname(convert(convert(n,base,10),`*`))

%p end proc:

%p select(mdr=4, [$1..10^5]); # _Robert Israel_, Oct 19 2015

%t Select[Range@ 800, FixedPoint[Times @@ IntegerDigits@ # &, #] == 4 &] (* _Michael De Vlieger_, Oct 19 2015 *)

%o (PARI) t(n) = {while(n>9, n=prod(i=1, #n=digits(n), n[i])); n};

%o for(n=0, 1e4, if(t(n) == 4, print1(n", "))); \\ _Altug Alkan_, Oct 19 2015

%Y Cf. A031347.

%Y Cf. A034048, A002275, A034049, A034050, A034051, A034052, A034053, A034054, A034055, A034056 (numbers having multiplicative digital roots 0-9).

%K nonn,base,easy

%O 1,1

%A _Patrick De Geest_, Sep 15 1998

%E Incorrect formula removed by _Martin Renner_, Oct 19 2015