A242739 Semiprimes having only straight digits.

4, 14, 74, 77, 111, 141, 177, 411, 417, 447, 471, 717, 771, 1111, 1114, 1141, 1147, 1174, 1177, 1411, 1417, 1441, 1477, 1711, 1714, 1717, 1774, 4117, 4141, 4171, 4174, 4411, 4414, 4417, 4471, 4474, 4711, 4714, 4717, 4741, 4747, 4771, 4777, 7111, 7114, 7117, 7141

Offset: 1

Author

K. D. Bajpai, May 21 2014

Keywords

• nonn
• base

Comments

A straight digit semiprime has only the straight digits, i.e., 1, 4 or 7.

Intersection of A001358 and A028373. - Michel Marcus, May 25 2014

Examples

			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.
		

Links

• K. D. Bajpai, Table of n, a(n) for n = 1..1727

Crossrefs

Cf. A001358, A028373, A079651, A108631, A108632.

Programs

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 *)