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 A002071 Number of pairs of consecutive integers x, x+1 such that all prime factors of both x and x+1 are at most the n-th prime. (Formerly M3386 N1366) 16
 1, 4, 10, 23, 40, 68, 108, 167, 241, 345, 482, 653, 869, 1153, 1502, 1930 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Størmer's theorem proves that a(n) is finite. - Charles R Greathouse IV, Feb 19 2013 Also: Number of positive integers x such that x(x+1) is prime(n)-smooth. - M. F. Hasler, Jan 16 2015 Also: Row lengths of A138180; partial sums of A145604. - M. F. Hasler, Jan 16 2015 REFERENCES N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence). N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence). LINKS E. F. Ecklund and R. B. Eggleton, Prime factors of consecutive integers, Amer. Math. Monthly, 79 (1972), 1082-1089. D. Eppstein, Smooth pairs. D. Eppstein, Python program D. H. Lehmer, On a problem of Størmer, Ill. J. Math., 8 (1964), 57-69. C. Stormer, Quelques théorèmes sur l'équation de Pell x^2 - Dy^2 = +-1 et leurs applications, Skrifter Videnskabs-selskabet (Christiania), Mat.-Naturv (1897). Kl. I (2). Wikipedia, Størmer's theorem FORMULA a(n) <= (2^n-1)*(prime(n)+1)/2 is implicit in Lehmer 1964. - Charles R Greathouse IV, Feb 19 2013 MATHEMATICA (* This program needs x maxima taken from A002072. *) xMaxima = A002072; smoothNumbers[p_, max_] := Module[{a, aa, k, pp, iter}, k = PrimePi[p]; aa = Array[a, k]; pp = Prime[Range[k]]; iter = Table[{a[j], 0, PowerExpand @ Log[pp[[j]], max/Times @@ (Take[pp, j-1]^Take[aa, j-1])]}, {j, 1, k}]; Table[Times @@ (pp^aa), Sequence @@ iter // Evaluate] // Flatten // Sort]; a[n_] := Module[{sn, cnt}, sn = smoothNumbers[Prime[n], xMaxima[[n]]+1]; cnt = 0; Do[If[sn[[i]]+1 == sn[[i+1]], cnt++], {i, 1, Length[sn]-1}]; cnt]; Table[an = a[n]; Print["a(", n, ") = ", an]; an, {n, 1, 16}] (* Jean-François Alcover, Nov 10 2016 *) A002072 = {1, 8, 80, 4374, 9800, 123200, 336140, 11859210, 11859210}; Table[Length[Select[Table[Max[FactorInteger[x], FactorInteger[x + 1]], {x, A002072[[n]]}], # <= Prime[n] &]], {n, 7}] (* Robert Price, Oct 29 2018 *) PROG (PARI) A002071(n)=[1, 4, 10, 23, 40, 68, 108, 167, 241, 345, 482, 653, 869, 1153, 1502][n] \\ "practical" solution. - M. F. Hasler, Jan 16 2015 (PARI) A002071(n, b=A002072, c=1, p=prime(n))={for(k=2, b(n), vecmax(factor(k++, p)[, 1])<=p && vecmax(factor(k--+(k<2), p)[, 1])<=p && c++); c} \\ b can be any upper bound for A002072, e.g., n->10^n should work, too. - M. F. Hasler, Jan 16 2015 CROSSREFS Cf. A002072, A145604, A145605, A145606. Cf. A138180 (triangle of x values for each n). Cf. A085152, A085153. Cf. A285283 (equivalent for x^2 + 1). - Tomohiro Yamada, Apr 22 2017 Sequence in context: A023378 A276308 A038423 * A024980 A002766 A305102 Adjacent sequences:  A002068 A002069 A002070 * A002072 A002073 A002074 KEYWORD nonn,nice,hard,more AUTHOR EXTENSIONS Better description and more terms from David Eppstein, Mar 23 2007 a(16) from Jean-François Alcover, Nov 10 2016 STATUS approved

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Last modified November 16 02:33 EST 2018. Contains 317252 sequences. (Running on oeis4.)