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A332121
a(n) = 2*(10^(2n+1)-1)/9 - 10^n.
12
1, 212, 22122, 2221222, 222212222, 22222122222, 2222221222222, 222222212222222, 22222222122222222, 2222222221222222222, 222222222212222222222, 22222222222122222222222, 2222222222221222222222222, 222222222222212222222222222, 22222222222222122222222222222, 2222222222222221222222222222222
OFFSET
0,2
FORMULA
a(n) = 2*A138148(n) + 1*10^n = A002276(2n+1) - 10^n.
G.f.: (1 + 101*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
A332121 := n -> 2*(10^(2*n+1)-1)/9-10^n;
MATHEMATICA
Array[2 (10^(2 # + 1)-1)/9 - 10^# &, 15, 0]
PROG
(PARI) apply( {A332121(n)=10^(n*2+1)\9*2-10^n}, [0..15])
(Python) def A332121(n): return 10**(n*2+1)//9*2-10**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. A332120 .. A332129 (variants with different middle digit 0, ..., 9).
Cf. A332131 .. A332191 (variants with different repeated digit 3, ..., 9).
Sequence in context: A344423 A238023 A204299 * A083962 A007942 A210257
KEYWORD
nonn,base,easy
AUTHOR
M. F. Hasler, Feb 09 2020
STATUS
approved