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A272306 Lesser of two consecutive semiprimes such that their sum is also semiprime. 4
4, 6, 25, 34, 38, 39, 46, 51, 57, 65, 69, 77, 87, 93, 95, 106, 111, 118, 129, 133, 145, 146, 161, 166, 169, 177, 178, 187, 194, 201, 205, 206, 209, 213, 218, 221, 249, 262, 278, 291, 298, 305, 309, 314, 323, 334, 335, 341, 355, 361, 377, 381, 394, 395, 407, 422, 446, 447, 473 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

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

EXAMPLE

51 is in the sequence because 51 = 3*17, the next semiprime is 55 = 5*11, and 51+55 = 106 = 2*53, is also semiprime.

MATHEMATICA

s[n_] := PrimeOmega@n == 2; L = Select[Range@480, s]; Pick[ Most@L, s /@ (Plus @@@ Partition[L, 2, 1])] (* Giovanni Resta, Apr 25 2016 *)

PROG

(PARI) proxsem(n)=my(p=n, s, r); while(s==0, p++; if(bigomega(p)==2, s=1; r=p)); p

{for(i=1, 500, if(bigomega(i)==2, a=proxsem(i); if(bigomega(a+i)==2, print1(i, ", "))))}

(PARI) issemi(n)=bigomega(n)==2

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(issemi(v[i+1]+v[i]), listput(u, v[i]))); Set(u) \\ Charles R Greathouse IV, Apr 29 2016

CROSSREFS

Cf. A001358, A272307, A272308, A272309.

Sequence in context: A009459 A239625 A123055 * A294996 A176858 A182333

Adjacent sequences:  A272303 A272304 A272305 * A272307 A272308 A272309

KEYWORD

nonn

AUTHOR

Antonio Roldán, Apr 25 2016

STATUS

approved

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Last modified January 23 00:58 EST 2018. Contains 298093 sequences.