A272309 Lesser of two consecutive semiprimes with a prime difference.

4, 6, 22, 26, 35, 39, 46, 49, 55, 62, 69, 74, 77, 82, 91, 95, 106, 115, 119, 134, 143, 155, 159, 161, 166, 178, 183, 185, 187, 194, 203, 206, 215, 219, 221, 235, 247, 254, 259, 262, 265, 267, 287, 289, 295, 299, 303, 309, 314, 319, 321, 323, 327, 329, 339, 341, 355, 358, 362

Offset

1,1

Links

Charles R Greathouse IV, Table of n, a(n) for n = 1..10000

Example

39 is in the sequence because 39 = 3*13, the next semiprime is 46 = 2*23, and 46-39 = 7 is prime.

Mathematica

L=Select[Range@370, PrimeOmega@# == 2 &]; Pick[Most@L, PrimeQ@ Differences@ L] (* Giovanni Resta, Apr 25 2016 *)

Prog

(PARI) proxsem(n)=local(p, s, r); s=0; p=n; while(s==0, p+=1; if(bigomega(p)==2, s=1; r=p)); p
{for(i=1, 400, if(bigomega(i)==2, a=proxsem(i)-i; if(isprime(a), print1(i, ", "))))}
(PARI) list(lim)=my(v=List(), u=v); lim\=1; while(bigomega(lim++)!=2, ); forprime(p=2, lim\2, forprime(q=2, lim\p, listput(v, p*q))); v=Set(v); for(i=1, #v-1, if(isprime(v[i+1]-v[i]), listput(u, v[i]))); Set(u) \\ Charles R Greathouse IV, Apr 29 2016

Crossrefs

Cf. A001358, A272306, A272307, A272308.

Sequence in context: A002270 A286358 A088228 * A108636 A101143 A083157

Adjacent sequences: A272306 A272307 A272308 * A272310 A272311 A272312

Keyword

nonn

Author

Antonio Roldán, Apr 25 2016

Status

approved