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A221651 Numbers divisible by their first digit squared (excluding those whose first digit is 1). 4
20, 24, 28, 36, 48, 50, 200, 204, 208, 212, 216, 220, 224, 228, 232, 236, 240, 244, 248, 252, 256, 260, 264, 268, 272, 276, 280, 284, 288, 292, 296, 306, 315, 324, 333, 342, 351, 360, 369, 378, 387, 396, 400, 416, 432, 448, 464, 480, 496, 500, 525, 550, 575 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

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

LINKS

Christian N. K. Anderson, Table of n, a(n) for n = 1..10000

Christian N. K. Anderson, Ulam Spiral for the first 10000 terms. [Draw the usual Ulam spiral, containing all the nonnegative integers, and color the numbers belonging to this sequence yellow]

EXAMPLE

48 is divisible by 4^2.

PROG

(R)

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

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

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

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

CROSSREFS

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

Sequence in context: A250289 A114843 A174629 * A108373 A109131 A171696

Adjacent sequences:  A221648 A221649 A221650 * A221652 A221653 A221654

KEYWORD

nonn,base

AUTHOR

Kevin L. Schwartz and Christian N. K. Anderson, May 04 2013

STATUS

approved

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Last modified December 2 05:01 EST 2021. Contains 349437 sequences. (Running on oeis4.)