A240918 Primes p such that p +/- product_of_digits(p) are both semiprimes.
211, 257, 269, 461, 463, 467, 523, 547, 769, 829, 839, 947, 967, 983, 1129, 1213, 1259, 1327, 1361, 1381, 1429, 1433, 1453, 1487, 1619, 1667, 1721, 1723, 1741, 1811, 1847, 2143, 2153, 2161, 2243, 2251, 2311, 2339, 2357, 2371, 2473, 2531, 2591, 2593, 2617, 2659
Offset: 1
Examples
211 is in the sequence because it is prime, and because 211 + (2 * 1 * 1) = 213 = 3 * 71 and 211 - (2 * 1 * 1) = 209 = 11 * 19 both are semiprimes. 461 is in the sequence because it is prime, and because 461 + (4 * 6 * 1) = 485 = 5 * 97 and 461 - (4 * 6 * 1) = 437 = 19 * 23 both are semiprimes.
Links
- K. D. Bajpai, Table of n, a(n) for n = 1..10460
Programs
-
Mathematica
Select[Prime[Range[500]], PrimeOmega[(Times @@ IntegerDigits[#] + #)] == 2 && PrimeOmega[(Times @@ IntegerDigits[#] - #)] == 2 &]
-
PARI
forprime(p=10,10^4,d=digits(p);pp=prod(i=1,#d,d[i]);if(bigomega(p+pp)==2&&bigomega(p-pp)==2,print1(p,", "))) \\ Derek Orr, Aug 02 2014