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A144062
a(1)=1; for n>1, a(n) = least integer > a(n-1) such that a(n)^2-a(n-1)^2 = semiprime
1
1, 4, 5, 8, 11, 18, 23, 30, 37, 42, 43, 44, 57, 58, 69, 80, 81, 86, 93, 94, 97, 100, 101, 102, 103, 108, 109, 110, 111, 116, 123, 124, 125, 132, 133, 134, 137, 140, 143, 144, 145, 146, 165, 172, 175, 178, 181, 186, 193, 196, 197, 198, 203, 204, 215, 218, 219
OFFSET
1,2
LINKS
EXAMPLE
2^2-1=3, not semiprime; 3^2-1=8, not semiprime; 4^2-1=15=3*5, semiprime, hence a(2)=4.
MATHEMATICA
sp[n_]:=Module[{k=n+1}, While[PrimeOmega[k^2-n^2]!=2, k++]; k]; NestList[ sp, 1, 60] (* Harvey P. Dale, Oct 18 2016 *)
PROG
(PARI) lista(nn) = {cura = 1; print1(cura, ", "); for (n=1, nn, nexta = cura + 1; while (bigomega(nexta^2-cura^2) != 2, nexta++); cura = nexta; print1(nexta, ", "); ); } \\ Michel Marcus, Feb 28 2014
CROSSREFS
Cf. A001358.
Sequence in context: A354937 A256535 A249669 * A066233 A130814 A183181
KEYWORD
easy,nonn
AUTHOR
Philippe Lallouet (philip.lallouet(AT)orange.fr), Sep 09 2008
EXTENSIONS
Corrected and extended by Michel Marcus, Feb 28 2014
STATUS
approved