A118725 -id:A118725 - cp's OEIS frontend

A321420 Primes p whose reversal is a Chen prime.

2, 3, 5, 7, 11, 13, 17, 31, 71, 73, 101, 107, 113, 131, 149, 157, 167, 179, 181, 191, 199, 311, 347, 353, 359, 389, 701, 733, 739, 743, 751, 761, 787, 797, 919, 941, 953, 967, 971, 983, 991, 1009, 1021, 1031, 1061, 1091, 1097, 1103, 1109, 1151, 1153, 1217, 1223

Offset: 1

Paolo Galliani, Nov 09 2018

73 is the smallest non-Chen prime whose reversal is a Chen prime.

			73 is in the sequence because its reversal is 37 which is a Chen prime (because 37 + 2 = 39 has at most two prime factors).

Cf. A118725, A109611, A102540.

cpQ[n_] := Module[{rev = FromDigits[Reverse[IntegerDigits[n]]]}, PrimeQ[rev] && PrimeOmega[rev + 2] < 3]; Select[Prime[Range[400]], cpQ] (* Amiram Eldar, Nov 09 2018 after Harvey P. Dale at A118725 *)

is(n) = if(isprime(n), rn = fromdigits(Vecrev(digits(n))); return(isprime(rn) && bigomega(rn+2) <= 2), 0) \\ David A. Corneth, Nov 09 2018

A321422 Primes that are non-Chen primes whose reversal is a Chen prime.

Original entry on oeis.org

73, 733, 739, 967, 1021, 1103, 1153, 1223, 1237, 1453, 1523, 1657, 1723, 1741, 1831, 3023, 3911, 7057, 7187, 7523, 7577, 7643, 7673, 7699, 7717, 7841, 9161, 9173, 9293, 9497, 9679, 9749, 9803, 9833, 9883, 9923, 9931, 10151, 10273, 10453, 10853, 11003, 11083, 11243, 11423

Offset: 1

Paolo Galliani, Nov 09 2018

The second term of the sequence: 733 is congruent to 1 mod 61, to 2 mod 43, to 3 mod 73, where 61 is the second non-Chen prime, 43 is the first non-Chen prime and 73 is the third non-Chen prime. 733 is also congruent to 4 mod (3^6).

Cf. A109611, A102540.

cpQ[n_] := Module[{rev = FromDigits[Reverse[IntegerDigits[n]]]}, PrimeOmega[n + 2] > 2 && PrimeQ[rev] && PrimeOmega[rev + 2] < 3]; Select[Prime[Range[1300]], cpQ] (* Amiram Eldar, Nov 09 2018 after Harvey P. Dale at A118725 *)

forprime(p=1,10^4,w=Vecrev(digits(p));q=0;for(j=1,length(w),q=q*10+w[j]);if(ispseudoprime(q)==1,if(bigomega(p+2)>2,if(bigomega(q+2)<=2,print1(p,", ")))))

cp's OEIS Frontend

A321420 Primes p whose reversal is a Chen prime.

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