login
A224400
Composite numbers that become prime when their digits are put in nondecreasing order.
2
20, 30, 32, 50, 70, 74, 76, 91, 92, 95, 98, 110, 130, 170, 172, 175, 176, 190, 194, 200, 203, 209, 217, 230, 232, 272, 275, 290, 292, 296, 300, 301, 302, 310, 319, 320, 322, 323, 329, 332, 370, 371, 374, 376, 391, 392, 394, 395, 398, 407, 437, 470, 473, 475
OFFSET
1,1
COMMENTS
Conjecture: a(n) ~ 7.75*n.
LINKS
Christian N. K. Anderson, Table of n, a(n) for n = 1..10000
EXAMPLE
217 = 7*31, but 127 is prime. 302 = 2*151, but 23 is prime.
MATHEMATICA
Select[Range[500], ! PrimeQ[#] && PrimeQ[FromDigits[Sort[IntegerDigits[#]]]] &] (* T. D. Noe, Apr 05 2013 *)
PROG
(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]]))
if(isprime(paste(x[x>0], collapse=""))>0) y=c(y, j) #drop leading zeros
}
}
CROSSREFS
Subset of A007935.
Sequence in context: A338158 A107714 A029721 * A132067 A072989 A216603
KEYWORD
nonn,base
STATUS
approved