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A182150
Semiprimes that are also semiprime when their digits are sorted into nondecreasing order.
2
4, 6, 9, 14, 15, 22, 25, 26, 33, 34, 35, 38, 39, 46, 49, 51, 55, 57, 58, 62, 69, 77, 85, 93, 94, 111, 115, 118, 119, 122, 123, 129, 133, 134, 143, 145, 146, 155, 158, 159, 166, 169, 177, 178, 185, 187, 202, 205, 206, 213, 219, 221, 226, 235, 237, 247, 249, 253
OFFSET
1,1
COMMENTS
This is to A211654 primes that are also prime when their digits are sorted into nondecreasing order as A001358 semiprimes are to A000040 primes. There is an ambiguity arising from OEIS conventions, exemplified by the semiprime 303, which sorts to 033 and truncates to the semiprime 33.
LINKS
EXAMPLE
51 is in the sequence because, though it is a semiprime whose digits are in descending order, once the digits are sorted to be nondecreasing, it is the semiprime 15, whose digits are (left to right) nondecreasing.
MAPLE
h:= proc(m) local k; for k from m+1 while isprime(k) or
add(i[2], i=ifactors(k)[2])<>2 do od; k
end:
a:= proc(n) option remember; local k, l, s;
k:= h(a(n-1));
do l:= sort(convert(k, base, 10));
s:= add(l[i]*10^(nops(l)-i), i=1..nops(l));
if h(s-1)=s then return k else k:=h(k) fi
od
end: a(0):=0:
seq(a(n), n=1..100); # Alois P. Heinz, Apr 22 2012
MATHEMATICA
Select[Range[300], PrimeOmega[#]==2&&PrimeOmega[FromDigits[ Sort[ IntegerDigits[ #]]]]==2&] (* Harvey P. Dale, Nov 13 2014 *)
CROSSREFS
KEYWORD
nonn,base,easy
AUTHOR
Jonathan Vos Post, Apr 18 2012
EXTENSIONS
More terms from Alois P. Heinz, Apr 22 2012
STATUS
approved