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 A145824 Lower twin primes p1 such that p1-1 is a square. 3
 5, 17, 101, 197, 5477, 8837, 16901, 17957, 21317, 25601, 52901, 65537, 106277, 115601, 122501, 164837, 184901, 193601, 220901, 341057, 401957, 470597, 490001, 495617, 614657, 739601, 846401, 972197, 1110917, 1144901, 1336337, 1464101 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS 3 is the only lower twin prime for which p1+1 is a square. This follows from the fact that lower twin primes > 3 are of the form 3n+2 and adding 1 we get a number of the form 3m. Then 3m = k^2 implies k = 3r and 3m = 9r^2. Subtracting 1 we have 3m = (3r-1)(3r+1) not prime contradicting 3m-1 is prime. Conjecture: There exist an infinite number of primes of this form. a(n) = A080149(n)^2 + 1. - Zak Seidov, Oct 21 2008 LINKS Zak Seidov, Table of n, a(n) for n=1..4663, a(n)<10^12 EXAMPLE p1 = 5 is a lower twin prime. 5-1 = 4 is a square. MATHEMATICA lst={}; Do[p=Prime@n; If[PrimeQ@(p+2)&&Sqrt[p-1]==IntegerPart[Sqrt[p-1]], AppendTo[lst, p]], {n, 9!}]; lst (* Vladimir Joseph Stephan Orlovsky, Aug 11 2009 *) PROG (PARI) g(n) = for(x=1, n, y=twinl(x)-1; if(issquare(y), print1(y+1", "))) twinl(n) = local(c, x); c=0; x=1; while(c

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Last modified July 28 17:15 EDT 2021. Contains 346335 sequences. (Running on oeis4.)