A224402 Composite numbers that become prime when their digits are put in nonincreasing order.

14, 16, 34, 35, 38, 112, 118, 119, 121, 124, 125, 128, 133, 134, 136, 142, 143, 145, 146, 152, 154, 164, 166, 175, 176, 182, 188, 194, 214, 215, 218, 314, 316, 334, 341, 343, 344, 346, 356, 358, 361, 364, 365, 368, 374, 377, 385, 386, 388, 395, 398, 412, 413

Offset: 1

Author

Christian N. K. Anderson and Kevin L. Schwartz, Apr 05 2013

Keywords

• nonn
• base

Comments

Because any number ending in zero is composite, the sequence experiences gaps of at least order O(a(n))-1 between changes in the most significant digit.

Examples

			194=2*97, but 941 is prime.
		

Links

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

Crossrefs

Subset of A007935.

Cf. A068653, A076055, A224400.

Programs

Mathematica

Select[Range[500], ! PrimeQ[#] && PrimeQ[FromDigits[Reverse[Sort[IntegerDigits[#]]]]] &] (* T. D. Noe, Apr 05 2013 *)

R

j=1; y=c(); library(gmp)
while(length(y)<1000) {
if(isprime((j=j+1))==0) {
x=sort(as.numeric(strsplit(as.character(j),spl="")[[1]]),decr=T)
if(isprime(paste(x,collapse=""))>0) y=c(y,j)
}
}