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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 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

LINKS

Harvey P. Dale, Table of n, a(n) for n = 1..1000

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: A117573 A256535 A249669 * A066233 A130814 A183181

Adjacent sequences:  A144059 A144060 A144061 * A144063 A144064 A144065

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

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Last modified July 15 16:09 EDT 2019. Contains 325049 sequences. (Running on oeis4.)