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 A210520 Number of cuban primes < 10^(n/2). 0
 0, 0, 1, 2, 4, 6, 11, 17, 28, 42, 64, 105, 173, 267, 438, 726, 1200, 2015, 3325, 5524, 9289, 15659, 26494, 44946, 76483, 129930, 221530, 377856, 645685, 1105802, 1895983, 3254036, 5593440, 9625882, 16578830, 28590987, 49347768, 85253634 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,4 COMMENTS A cuban prime has the form (x+1)^3 - x^3, which equals 3x*(x+1) + 1 (A002407). LINKS FORMULA a(2*n) = A113478(n). - Andrew Howroyd, Jan 14 2020 EXAMPLE As the smallest cuban primes equal to the difference of two consecutive cubes p = (x+1)^3 - x^3, is 7 for x = 1, and as floor (10^(1/2)) = 3, a(0) = a(1) = 0 and a(2) = 1. MATHEMATICA cnt = 0; nxt = 1; t = {0}; Do[p = 3*k*(k + 1) + 1; If[p > nxt, AppendTo[t, cnt]; nxt = nxt*Sqrt[10]]; If[PrimeQ[p], cnt++], {k, 100000}]; t (* T. D. Noe, Jan 29 2013 *) PROG (PARI) b(n)={my(s=0, k=0, t=1); while(t<=n, s+=isprime(t); k++; t += 6*k); s} a(n)={b(sqrtint(10^n))} \\ Andrew Howroyd, Jan 14 2020 CROSSREFS Cf. A002407, A113478, A221794. Sequence in context: A336134 A255214 A222047 * A018144 A115315 A004698 Adjacent sequences: A210517 A210518 A210519 * A210521 A210522 A210523 KEYWORD nonn AUTHOR Vladimir Pletser, Jan 26 2013 EXTENSIONS a(31)-a(37) from Andrew Howroyd, Jan 14 2020 STATUS approved

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Last modified April 2 02:53 EDT 2023. Contains 361723 sequences. (Running on oeis4.)