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 A332174 a(n) = 7*(10^(2n+1)-1)/9 - 3*10^n. 1
 4, 747, 77477, 7774777, 777747777, 77777477777, 7777774777777, 777777747777777, 77777777477777777, 7777777774777777777, 777777777747777777777, 77777777777477777777777, 7777777777774777777777777, 777777777777747777777777777, 77777777777777477777777777777, 7777777777777774777777777777777 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,1 COMMENTS See A183179 = {2, 3, 6, 23, 36, 69, 561, ...} for the indices of primes. LINKS Makoto Kamada, Factorization of 77...77477...77, updated Dec 11 2018. Index entries for linear recurrences with constant coefficients, signature (111,-1110,1000). FORMULA a(n) = 7*A138148(n) + 4*10^n. G.f.: (4 + 303*x - 1000*x^2)/((1 - x)*(1 - 10*x)*(1 - 100*x)). a(n) = 111*a(n-1) - 1110*a(n-2) + 1000*a(n-3) for n>2. E.g.f.: (1/9)*exp(x)*(70*exp(99*x) - 27*exp(9*x) - 7). - Stefano Spezia, Feb 08 2020 MAPLE A332174 := n -> 7*(10^(n*2+1)-1)/9 - 3*10^n; MATHEMATICA Array[7 (10^(2 # + 1) - 1)/9 - 3*10^# &, 15, 0] PROG (PARI) apply( {A332174(n)=10^(n*2+1)\9*7-3*10^n}, [0..15]) (Python) def A332174(n): return 10**(n*2+1)//9*7-3*10^n CROSSREFS Cf. A138148 (cyclops numbers with binary digits only). Cf. (A077781-1)/2 = A183179: indices of primes. Cf. A002275 (repunits R_n = (10^n-1)/9), A002281 (7*R_n), A011557 (10^n). Cf. A332171 .. A332179 (variants with different middle digit 1, ..., 9). Sequence in context: A222961 A160737 A128846 * A195625 A268838 A292306 Adjacent sequences:  A332171 A332172 A332173 * A332175 A332176 A332178 KEYWORD nonn,base,easy AUTHOR M. F. Hasler, Feb 08 2020 STATUS approved

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Last modified September 28 05:33 EDT 2020. Contains 337392 sequences. (Running on oeis4.)