The OEIS is supported by the many generous donors to the OEIS Foundation.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A219340 Numbers not multiples of 9 whose digital sum coincides with digital sum of their largest proper divisor. 3

%I #12 May 13 2013 01:50:09

%S 361,551,703,1007,1273,1691,1843,2033,2071,2183,2413,2603,2641,2701,

%T 2831,2923,3071,3173,3293,3743,3781,4033,4313,4351,4541,5143,5263,

%U 5513,6023,6031,6401,6403,6623,6631,6821,7081,7141,7363,7391,7543,8303,8341,8531

%N Numbers not multiples of 9 whose digital sum coincides with digital sum of their largest proper divisor.

%C The number n has a prime factor of the form 18k+1.

%H Antonio Roldán and T. D. Noe, <a href="/A219340/b219340.txt">Table of n, a(n) for n = 1..10000</a> (542 terms from Antonio Roldán)

%e 12673 is in the sequence because 12673 = 19*23*29, their largest proper divisor is 667. Both have a digital sum of 19.

%t Select[Range[2, 10000], Mod[#, 9] > 0 && Total[IntegerDigits[#]] == Total[IntegerDigits[Divisors[#][[-2]]]] &] (* _T. D. Noe_, Nov 20 2012 *)

%o (PARI) largdiv(n)=if(n==1, 1, n/factor(n)[1, 1]) /* _Charles R Greathouse IV_, Jun 15 2011 */

%o digsum(n)={local (d,p); d=0; p=n; while(p,d+=p%10;p=floor(p/10)); return(d)}

%o { k=0; for (n=2, 10^5, if(digsum(n)==digsum(largdiv(n))&&n%9>0, k=k+1;write("B219340.txt",k,", ",n))); } /* Antonio Roldán, Nov 18 2012 */

%K nonn,base

%O 1,1

%A _Antonio Roldán_, Nov 18 2012

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recents
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified September 23 19:57 EDT 2023. Contains 365554 sequences. (Running on oeis4.)