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A272307 Lesser of two consecutive semiprimes such that their difference is also semiprime. 3
10, 15, 51, 58, 65, 87, 111, 123, 129, 146, 209, 226, 237, 249, 274, 278, 291, 305, 335, 346, 365, 371, 377, 382, 403, 407, 427, 447, 454, 485, 489, 493, 497, 505, 529, 538, 545, 573, 591, 597, 629, 635, 649, 681, 699, 707, 713, 749, 767, 781, 785, 803, 807, 831, 843, 889, 901 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

Table of n, a(n) for n=1..57.

EXAMPLE

65 is in the sequence because 65 = 5*13, the next semiprime is 69 = 3*23, and 69-65 = 4 = 2*2, is also semiprime.

MATHEMATICA

s[n_] := PrimeOmega@n==2; L=Select[Range@910, s]; Pick[ Most@L, s /@ 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, 1000, if(bigomega(i)==2, a=proxsem(i); if(bigomega(a-i)==2, print1(i, ", "))))}

CROSSREFS

Cf. A001358, A272306, A272308, A272309.

Sequence in context: A212794 A048061 A188651 * A092192 A119039 A047189

Adjacent sequences:  A272304 A272305 A272306 * A272308 A272309 A272310

KEYWORD

nonn

AUTHOR

Antonio Roldán, Apr 25 2016

STATUS

approved

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Last modified January 21 13:51 EST 2018. Contains 297994 sequences.