This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
23 begins with a 2, so a(23) = 2.
a000030 = until (< 10) (`div` 10) -- Reinhard Zumkeller, Feb 20 2012, Feb 11 2011
[Intseq(n)[#Intseq(n)]: n in [1..100]]; // Vincenzo Librandi, Nov 17 2018
A000030 := proc(n) if n = 0 then 0; else convert(n,base,10) ; %[-1] ; end if; end proc: seq(A000030(n),n=0..200) ;# N. J. A. Sloane, Feb 10 2017
Join[{0},First[IntegerDigits[#]]&/@Range[90]] (* Harvey P. Dale, Mar 01 2011 *)
Table[Floor[n/10^(Floor[Log10[n]])], {n, 1, 50}] (* G. C. Greubel, May 16 2017 *)
Table[NumberDigit[n,IntegerLength[n]-1],{n,0,100}] (* Harvey P. Dale, Aug 29 2021 *)
a(n)=if(n<10,n,a(n\10)) \\ Mainly for illustration.
A000030(n)=n\10^logint(n+!n,10) \\ Twice as fast as a(n)=digits(n)[1]. Before digits() was added in PARI v.2.6.0 (2013), one could use, e.g., Vecsmall(Str(n))[1]-48. - M. F. Hasler, Nov 17 2018
def a(n): return int(str(n)[0]) print([a(n) for n in range(85)]) # Michael S. Branicky, Jul 01 2022
a[1]=1;a[n_]:=a[n]=Block[{k=1},While[MemberQ[s=Array[a,n-1],k]||Or@@(#<#2<#3&@@@Partition[Flatten[IntegerDigits/@Join[s[[-2;;]],{k}]],3,1]),k++];k];Array[a,126] (* Giorgos Kalogeropoulos, May 13 2022 *)
is_ok = lambda s: not any(s[i-2] < s[i-1] < s[i] for i in range(2, len(s)))
terms, appears, digits = [1],{1},'1'
for i in range(100):
t = 1
while not(t not in appears and is_ok(digits + str(t))):
t += 1
terms.append(t); appears.add(t); digits = digits + str(t)
digits = digits[-2:]
print(terms) # Gleb Ivanov, Dec 04 2021
import Data.List (delete, intersect); import Data.Function (on)
a184992 n = a184992_list !! (n-1)
a184992_list = 1 : f 1 [2..] where
f u vs = v : f v (delete v vs)
where v : _ = filter (not . null . (intersect `on` show) u) vs
-- Reinhard Zumkeller, Jul 01 2013
FromDigits /@ Nest[Function[a, Append[a, Block[{k = 2, d}, While[Nand[FreeQ[a, #], IntersectingQ[a[[-1]], #]] &@ Set[d, IntegerDigits@ k], k++]; d]]], {{1}}, 73] (* Michael De Vlieger, Mar 17 2018 *)
A184992(n,show=0)={my(a=1,u=2^1);for(k=2,n,show && print1(a",");a=Set(Vec(Str(a))); for(j=2,9e9,bittest(u,j) && next;setintersect(Set(Vec(Str(j))),a) || next; u+=2^a=j; break));a} \\ M. F. Hasler, Dec 22 2011
from itertools import count, islice
def agen(): # generator of terms
an, aset, mink = 1, {1}, 1
while True:
yield an
digset = set(str(an))
an = next(k for k in count(mink) if k not in aset and set(str(k))&digset)
aset.add(an)
while mink in aset: mink += 1
print(list(islice(agen(), 74))) # Michael S. Branicky, Oct 03 2024
a(11)=12 since a(10)=10 and 12 is the smallest number not occurring earlier in the sequence that contains a digit (2) that is not in 10.
isok(k, prev) = {my(d=digits(k)); for (i=1, #d, if (!vecsearch(prev, d[i]), return(1));); return(0);}
find(va, n) = {my(k=1, prev=Set(digits(va[n-1]))); while (vecsearch(Set(va), k) || !isok(k, prev), k++); k;}
lista(nn) = {my(va = vector(nn)); va[1] = 1; for (n=2, nn, va[n] = find(va, n);); va;} \\ Michel Marcus, May 11 2022
(C++) See Links section.
from itertools import count, islice
def agen(): # generator of terms
an, aset, mu, mink = 0, set(), [10, 1, 2, 3, 4, 5, 6, 7, 8, 9], 1
while set(str(an)) != set("0123456789"):
notin = set("0123456789") - set(str(an))
an = min(mu[i] for i in range(10) if str(i) in notin)
yield an; aset.add(an)
for i in range(10): # update min unused containing digit i
while mu[i] in aset or str(i) not in str(mu[i]): mu[i] += 1
for k in range(mink, min(mu)): aset.discard(k)
mink = min(mu)
print(list(islice(agen(), 67))) # Michael S. Branicky, Aug 26 2022
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