# [seqfan] Re: "Double and Reverse" chains of primes

Felix Fröhlich felix.froe at gmail.com
Fri Nov 4 16:45:44 CET 2016

```Here is a list of all such chains of length > 5 less than 10^8:

[389047, 490877, 457189, 873419, 8386471, 24927761]
[3632297, 4954627, 4529099, 8918509, 81073871, 247741261]
[3733637, 4727647, 4925549, 8901589, 87130871, 247162471]
[3957097, 4914197, 4938289, 8756789, 87531571, 241360571]
[36125927, 45815227, 45403619, 83270809, 816145661, 2231922361]
[36225347, 49605427, 45801299, 89520619, 832140971, 2491824661]
[36567977, 45953137, 47260919, 83812549, 890526761, 2253501871]
[36610127, 45202237, 47440409, 81808849, 896716361, 2272343971]
[36842537, 47058637, 47271149, 89224549, 890944871, 2479881871]
[36956897, 49731937, 47836499, 89927659, 813558971, 2497117261]
[37144577, 45198247, 49469309, 81683989, 879763361, 2276259571]
[38839837, 47697677, 45359359, 81781709, 814365361, 2270378261]
[38849887, 47799677, 45399559, 81199709, 814993261, 2256899261]
[39271487, 47924587, 47194859, 81798349, 896695361, 2270933971]
[39370937, 47814787, 47592659, 81358159, 813617261, 2254327261]
[39439747, 49497887, 47759989, 87991559, 811389571, 2419772261]
[39619787, 47593297, 49568159, 81363199, 893627261, 2254527871]

(PARI) db_rev(n) = eval(concat(Vecrev(Str(2*n))))
chain(n) = my(v=[n], k=db_rev(n)); while(ispseudoprime(k), v=concat(v, k);
k=db_rev(k)); v
forprime(p=2, , my(v=chain(p)); if(#v > 5, print(v)))

Best regards
Felix

2016-11-03 20:55 GMT+01:00 Zak Seidov via SeqFan <seqfan at list.seqfan.eu>:

> "Double and Reverse" chains of primes
> {389047, 490877, 457189, 873419, 8386471, 24927761}
> {3632297, 4954627, 4529099, 8918509, 81073871, 247741261}
> {3733637, 4727647, 4925549, 8901589, 87130871, 247162471}
> {3957097, 4914197, 4938289, 8756789, 87531571, 241360571}
> --
> Zak  Seidov
>
> --
> Seqfan Mailing list - http://list.seqfan.eu/
>

```