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A332172
a(n) = 7*(10^(2n+1)-1)/9 - 5*10^n.
1
2, 727, 77277, 7772777, 777727777, 77777277777, 7777772777777, 777777727777777, 77777777277777777, 7777777772777777777, 777777777727777777777, 77777777777277777777777, 7777777777772777777777777, 777777777777727777777777777, 77777777777777277777777777777, 7777777777777772777777777777777
OFFSET
0,1
COMMENTS
Indices of prime terms: {0, 1, 3, 7, 10, 12, 480, 949, ...} = A183178.
FORMULA
a(n) = 7*A138148(n) + 2*10^n.
G.f.: (2 + 505*x - 1200*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.
MAPLE
A332172 := n -> 7*(10^(n*2+1)-1)/9 -5*10^n;
MATHEMATICA
Array[7 (10^(2 # +1)-1)/9 -5*10^# &, 15, 0]
PROG
(PARI) apply( {A332172(n)=10^(n*2+1)\9*7-5*10^n}, [0..25])
(Python) def A332172(n): return 10**(n*2+1)//9*7-5*10^n
CROSSREFS
Cf. A138148 (cyclops numbers with binary digits only).
Cf. A332171 (analog with middle digit 1).
Cf. (A077777-1)/2 = A183178: indices of primes.
Cf. A002275 (repunits R_n = [10^n/9]), A002281 (7*R_n), A011557 (10^n).
Cf. A332171 .. A332179 (variants with different middle digit 1, ..., 9).
Sequence in context: A053600 A090275 A090565 * A072384 A109949 A375956
KEYWORD
nonn,base,easy
AUTHOR
M. F. Hasler, Feb 06 2020
STATUS
approved