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 A247250 Indices of Pell numbers having exactly one primitive prime factor. 0
 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 18, 21, 24, 29, 30, 32, 33, 35, 38, 41, 42, 50, 53, 54, 56, 58, 59, 66, 69, 89, 90, 94, 95, 97, 99, 101, 104, 117, 118, 120, 135, 138, 160, 167, 181, 191, 210, 221, 237, 242, 247 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Conjecture: The n-th Pell number A000129(n) has a primitive prime factor for all n > 1. (The n-th Fibonacci number A000045(n) has a primitive prime factor for all n except n = 0, 1, 2, 6, and 12.) For prime p, all prime factors of Pell(p) are primitive. Hence the only primes in this sequence are the prime numbers in A096650, which gives the indices of prime Pell numbers. LINKS EXAMPLE Pell(1) = 1, which has no prime factors, so 1 is not in this sequence. Pell(4) = 12 = 2^2 * 3, but 2 is not a primitive prime factor, and 3 is the only primitive prime factor of Pell(4), so 4 is in this sequence. Pell(5) = 29, which is a prime and the only primitive prime factor of itself, so 5 is in this sequence. Pell(12) = 13860 = 2^2 * 3^2 * 5 * 7 * 11, but none of 2, 3, 5, 7 is a primitive prime factor, and 11 is the only primitive prime factor of Pell(12), so 12 is in this sequence. Pell(14) = 80782 = 2 * 13^2 * 239, but neither 2 nor 13 is a primitive prime factor, and 239 is the only primitive prime factor of Pell(14), so 14 is in this sequence. Pell(19) = 6625109 = 37 * 179057, both of which are primitive prime factors of Pell(19), so 19 is not in this sequence. MATHEMATICA Select[Range[1000], PrimePowerQ[(1-Sqrt[2])^EulerPhi[#]*Cyclotomic[#, (1+Sqrt[2])/(1-Sqrt[2])]/GCD[Cyclotomic[#, (1+Sqrt[2])/(1-Sqrt[2])], # ]]&] - Eric Chen, Dec 12 2014 pell[n_] := pell[n] = ((1+Sqrt[2])^n-(1-Sqrt[2])^n )/(2*Sqrt[2]) // Round; primitivePrimeFactors[n_] := Cases[FactorInteger[pell[n]][[All, 1]], p_ /; And @@ (GCD[p, #] == 1 & /@ Array[pell, n-1])]; Reap[For[n=2, n <= 200, n++, If[Length[primitivePrimeFactors[n]] == 1, Print[n]; Sow[n]]]][[2, 1]] (* Jean-François Alcover, Dec 12 2014 *) PROG (PARI) pell(n) = imag((1 + quadgen(8))^n); isok(pf, vp) = sum(i=1, #pf, vecsearch(vp, pf[i]) == 0) == 1; lista(nn) = {vp = []; for (n=2, nn, pf = factor(pell(n))[, 1]; if (isok(pf, vp), print1(n, ", ")); vp = vecsort(concat(vp, pf), , 8); ); } \\ Michel Marcus, Nov 29 2014 CROSSREFS Cf. A152012 (for Fibonacci numbers). Cf. A000129, A246556, A096650, A086383, A008555, A175181. Sequence in context: A246089 A078510 A246100 * A017909 A345201 A316530 Adjacent sequences:  A247247 A247248 A247249 * A247251 A247252 A247253 KEYWORD nonn AUTHOR Eric Chen, Nov 29 2014 EXTENSIONS Two incorrect terms (72 and 110) deleted by Colin Barker, Nov 29 2014 More terms from Colin Barker, Nov 30 2014 STATUS approved

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Last modified June 17 22:14 EDT 2021. Contains 345086 sequences. (Running on oeis4.)