A225801 The only semiprime s such that 2*a < s < 2*b where a, b are consecutive semiprimes.

51, 69, 74, 77, 106, 111, 115, 129, 187, 226, 235, 237, 267, 291, 314, 355, 371, 403, 407, 411, 422, 427, 437, 497, 519, 533, 591, 597, 649, 655, 669, 713, 723, 763, 785, 789, 831, 893, 905, 939, 943, 965, 1003, 1027, 1037, 1059, 1067, 1073, 1126, 1159, 1165

Offset: 1

Author

Irina Gerasimova, Aug 18 2013

Keywords

• nonn

Examples

			74 is the only semiprime in the interval [2*35, 2*38] = [70, 76] where 35, 38 are consecutive semiprimes.
		

Links

• Alois P. Heinz, Table of n, a(n) for n = 1..1000

Crossrefs

Cf. A001358, A166251, A217561.

Programs

PARI

issemi(n)=bigomega(n)==2
precsemi(n)=while(!issemi(n--),); n
nextsemi(n)=while(!issemi(n++),); n
is(n)=if(!issemi(n),return(0)); my(a=precsemi(ceil(n/2)),b=nextsemi(n\2)); nextsemi(a)==b && nextsemi(2*a)==n && precsemi(2*b)==n \\ Charles R Greathouse IV, Mar 18 2014