OFFSET
1,1
COMMENTS
A straight digit semiprime has only the straight digits, i.e., 1, 4 or 7.
LINKS
K. D. Bajpai, Table of n, a(n) for n = 1..1727
EXAMPLE
471 = 3 * 157 is semiprime and has only straight digits 4, 7 and 1. Hence it is in the sequence.
1147 = 31 * 37 is semiprime and has only straight digits 1, 1, 4 and 7. Hence it is in the sequence.
MATHEMATICA
A242739 = {}; Do[a = PrimeOmega[n]; If [a == 2 && Intersection[IntegerDigits[n], {0, 2, 3, 5, 6, 8, 9}] == {}, AppendTo[A242739, n]], {n, 8000}]; A242739
Table[Select[FromDigits/@Tuples[{1, 4, 7}, n], PrimeOmega[#]==2&], {n, 4}]//Flatten (* Harvey P. Dale, Sep 23 2022 *)
CROSSREFS
KEYWORD
nonn,base
AUTHOR
K. D. Bajpai, May 21 2014
STATUS
approved