A221651 - OEIS

%I #37 May 20 2013 01:43:25

%S 20,24,28,36,48,50,200,204,208,212,216,220,224,228,232,236,240,244,

%T 248,252,256,260,264,268,272,276,280,284,288,292,296,306,315,324,333,

%U 342,351,360,369,378,387,396,400,416,432,448,464,480,496,500,525,550,575

%N Numbers divisible by their first digit squared (excluding those whose first digit is 1).

%C Numbers where  floor(n/10^floor(log(n)))^2 divides n.

%H Christian N. K. Anderson, <a href="/A221651/b221651.txt">Table of n, a(n) for n = 1..10000</a>

%H Christian N. K. Anderson, <a href="/A221651/a221651_1.gif">Ulam Spiral</a> for the first 10000 terms. [Draw the usual Ulam spiral, containing all the nonnegative integers, and color the numbers belonging to this sequence yellow]

%e 48 is divisible by 4^2.

%o (R)

%o x=0; y=rep(0,1000); len=0

%o firstdig<-function(x) as.numeric(substr(as.character(x),1,1))

%o isint<-function(x) x==as.integer(x)

%o while(len<10000) if((fd=firstdig((x=x+1)))>1) if(isint(x/fd^2)) y[(len=len+1)]=x

%Y Cf. A132359, A034709, A034837, A005349, A007602, A034838, A225297, A225722, A225683.

%K nonn,base

%O 1,1

%A _Kevin L. Schwartz_ and _Christian N. K. Anderson_, May 04 2013