A224400 Composite numbers that become prime when their digits are put in nondecreasing order.

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.

Cf. A068653, A076055, A224402.

Sequence in context: A338158 A107714 A029721 * A132067 A072989 A216603

Adjacent sequences: A224397 A224398 A224399 * A224401 A224402 A224403

Keyword

nonn,base

Author

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

Status

approved