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A332127
a(n) = 2*(10^(2n+1)-1)/9 + 5*10^n.
3
7, 272, 22722, 2227222, 222272222, 22222722222, 2222227222222, 222222272222222, 22222222722222222, 2222222227222222222, 222222222272222222222, 22222222222722222222222, 2222222222227222222222222, 222222222222272222222222222, 22222222222222722222222222222, 2222222222222227222222222222222
OFFSET
0,1
FORMULA
a(n) = 2*A138148(n) + 7*10^n = A002276(2n+1) + 5*10^n.
G.f.: (7 - 505*x + 300*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
A332127 := n -> 2*(10^(2*n+1)-1)/9+5*10^n;
MATHEMATICA
Array[2 (10^(2 # + 1)-1)/9 + 5*10^# &, 15, 0]
PROG
(PARI) apply( {A332127(n)=10^(n*2+1)\9*2+5*10^n}, [0..15])
(Python) def A332127(n): return 10**(n*2+1)//9*2+5*10**n
CROSSREFS
Cf. A002275 (repunits R_n = (10^n-1)/9), A002276 (2*R_n), A011557 (10^n).
Cf. A138148 (cyclops numbers with binary digits), A002113 (palindromes).
Cf. A332117 .. A332197 (variants with different repeated digit 1, ..., 9).
Cf. A332120 .. A332129 (variants with different middle digit 0, ..., 9).
Sequence in context: A100465 A140031 A066413 * A222942 A289634 A065581
KEYWORD
nonn,base,easy
AUTHOR
M. F. Hasler, Feb 09 2020
STATUS
approved