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A285470 Numbers k where "2" appears as the second digit of the decimal representation. 1
12, 22, 32, 42, 52, 62, 72, 82, 92, 120, 121, 122, 123, 124, 125, 126, 127, 128, 129, 220, 221, 222, 223, 224, 225, 226, 227, 228, 229, 320, 321, 322, 323, 324, 325, 326, 327, 328, 329, 420, 421, 422, 423, 424, 425, 426, 427, 428, 429, 520, 521, 522, 523, 524, 525, 526, 527, 528, 529, 620, 621, 622, 623, 624, 625, 626, 627 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

To find a(n), concatenate the first digit of n with 2 and then the other digits (if any) from n. See example. - David A. Corneth, Jun 12 2017

LINKS

David A. Corneth, Table of n, a(n) for n = 1..10000

FORMULA

From Robert Israel, Jun 12 2017: (Start)

a(10*n+j) = 10*a(n)+j for 0<=j<=9 and n >= 1.

G.f. g(x) satisfies g(x) = 10*(1-x^10)*g(x^10)/(1-x) + (x + 2*x + ... + 9*x^9)*x^10/(1-x^10) + 12*x + 22*x^2 + ... + 92*x^9. (End)

EXAMPLE

a(21) = 221, a(36) = 326.

As the first digit of 983 is 9, and the others are 83, a(983) = 9283. - David A. Corneth, Jun 12 2017

MAPLE

seq(seq(seq(a*10^d + 2*10^(d-1)+c, c=0..10^(d-1)-1), a=1..9), d=1..2); # Robert Israel, Jun 12 2017

MATHEMATICA

Table[FromDigits@ Apply[Join, {{First@ #}, {2}, Rest@ #}] &@ IntegerDigits@ n, {n, 67}] (* Michael De Vlieger, Jun 12 2017 *)

PROG

(PARI) isok(n) = (n>9) && digits(n)[2] == 2; \\ Michel Marcus, Jun 12 2017

(PARI) a(n) = my(d = digits(n)); fromdigits(concat([d[1], [2], vector(#d-1, i, d[i+1])])) \\ David A. Corneth, Jun 12 2017

(PARI) nxt(n) = {if(isok(n+1), n+1, d = digits(n); t = 9*10^(#d-2); if(d[1]==9, t*=3); n+=t++) \\ David A. Corneth, Jun 12 2017

(Python)

def a(n): s = str(n); return int(s[0] + "2" + s[1:])

print([a(n) for n in range(1, 68)]) # Michael S. Branicky, Dec 22 2021

CROSSREFS

Cf. A011532 (containing 2), A052404 (without 2), A217394 (starting with 2).

Sequence in context: A286094 A348187 A098955 * A124885 A115745 A129639

Adjacent sequences:  A285467 A285468 A285469 * A285471 A285472 A285473

KEYWORD

nonn,base,easy

AUTHOR

Jamie Robert Creasey, Apr 19 2017

STATUS

approved

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Last modified September 30 08:55 EDT 2022. Contains 357104 sequences. (Running on oeis4.)