cp's OEIS Frontend

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.

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A247813 Numbers in decimal representation with distinct digits, such that in Spanish their digits are in alphabetic order.

Original entry on oeis.org

0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 21, 23, 26, 27, 28, 29, 31, 41, 42, 43, 46, 47, 48, 49, 51, 52, 53, 54, 56, 57, 58, 59, 61, 63, 67, 71, 73, 81, 83, 86, 87, 91, 93, 96, 97, 98, 231, 261, 263, 267, 271, 273, 281, 283, 286, 287, 291, 293, 296, 297, 298, 421, 423
Offset: 1

Views

Author

Reinhard Zumkeller, Oct 05 2014

Keywords

Comments

List of decimal digits, alphabetically sorted by their names in Spanish:
0 cero, 5 cinco, 4 cuatro, 2 dos, 9 nueve, 8 ocho, 6 seis, 7 siete, 3 tres, 1 uno/una;
finite sequence with last and largest term a(512) = 542986731.

Crossrefs

Intersection of A010784 and A161390 .
Cf. A247800 (Czech), A247801 (Danish), A247802 (Dutch), A053433 (English), A247803 (Finnish), A247804 (French), A247805 (German), A247806 (Hungarian), A247807 (Italian), A247808 (Latin), A247809 (Norwegian), A247810 (Polish), A247807 (Portuguese), A247811 (Russian), A247812 (Slovak), A247809 (Swedish), A247814 (Turkish).

Programs

  • Haskell
    import Data.IntSet (fromList, deleteFindMin, union)
    import qualified Data.IntSet as Set (null)
    a247813 n = a247813_list !! (n-1)
    a247813_list = 0 : f (fromList [1..9]) where
       f s | Set.null s = []
           | otherwise  = x : f (s' `union`
             fromList (map (+ 10 * x) $ tail $ dropWhile (/= mod x 10) digs))
           where (x, s') = deleteFindMin s
       digs = [0, 5, 4, 2, 9, 8, 6, 7, 3, 1]

A247814 Numbers in decimal representation with distinct digits, such that in Turkish their digits are in alphabetic order.

Original entry on oeis.org

0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 13, 14, 17, 18, 19, 20, 23, 27, 28, 37, 40, 42, 43, 47, 48, 50, 51, 52, 53, 54, 57, 58, 59, 60, 61, 62, 63, 64, 65, 67, 68, 69, 80, 83, 87, 90, 92, 93, 94, 97, 98, 103, 107, 120, 123, 127, 128, 137, 140, 142, 143, 147
Offset: 1

Views

Author

Reinhard Zumkeller, Oct 05 2014

Keywords

Comments

List of decimal digits, alphabetically sorted by their names in Turkish:
6 altı, 5 beş, 1 bir, 9 dokuz, 4 dört, 2 iki, 8 sekiz, 0 sıfır, 3 üç, 7 yedi;
finite sequence with last and largest term a(1020) = 6519428037.

Crossrefs

Intersection of A010784 and A247764.
Cf. A247800 (Czech), A247801 (Danish), A247802 (Dutch), A053433 (English), A247803 (Finnish), A247804 (French), A247805 (German), A247806 (Hungarian), A247807 (Italian), A247808 (Latin), A247809 (Norwegian), A247810 (Polish), A247807 (Portuguese), A247811 (Russian), A247812 (Slovak), A247813 (Spanish), A247809 (Swedish).

Programs

  • Haskell
    import Data.IntSet (fromList, deleteFindMin, union)
    import qualified Data.IntSet as Set (null)
    a247814 n = a247814_list !! (n-1)
    a247814_list = 0 : f (fromList [1..9]) where
       f s | Set.null s = []
           | otherwise  = x : f (s' `union`
             fromList (map (+ 10 * x) $ tail $ dropWhile (/= mod x 10) digs))
           where (x, s') = deleteFindMin s
       digs = [6, 5, 1, 9, 4, 2, 8, 0, 3, 7]

A109303 Numbers k with at least one duplicate base-10 digit (A107846(k) > 0).

Original entry on oeis.org

11, 22, 33, 44, 55, 66, 77, 88, 99, 100, 101, 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 121, 122, 131, 133, 141, 144, 151, 155, 161, 166, 171, 177, 181, 188, 191, 199, 200, 202, 211, 212, 220, 221, 222, 223, 224, 225, 226, 227, 228, 229, 232, 233, 242
Offset: 1

Views

Author

Rick L. Shepherd, Jun 24 2005

Keywords

Comments

Complement of A010784, numbers with distinct base-10 digits, so all numbers greater than 9876543210 (last term of A010784) are terms. a(263)=1001 is the first term not also a term of A044959; a(264)=1002 is the first term not also a term of A084050. The terms of A044959 greater than 9 are a subsequence. The terms of A084050 greater than 90 are a subsequence.
A178788(a(n)) = 0; A178787(a(n)) = A178787(a(n)-1); A043537(a(n)) < A109303(a(n)). - Reinhard Zumkeller, Jun 30 2010
A227362(a(n)) < a(n). - Reinhard Zumkeller, Jul 09 2013

Crossrefs

Cf. A010784 (numbers with distinct digits), A044959 (numbers with no two equally numerous digits), A084050 (numbers with a palindromic permutation of digits), A107846 (number of duplicate digits of n). Also see A062813, which gives the largest number in each base containing all distinct digits.

Programs

  • Haskell
    a109303 n = a109303_list !! (n-1)
    a109303_list = filter ((> 0) . a107846) [0..]
    -- Reinhard Zumkeller, Jul 09 2013
    
  • Mathematica
    Select[Range[300], Max[DigitCount[#]] > 1 &] (* Harvey P. Dale, Jan 14 2011 *)
  • Python
    def ok(n): s = str(n); return len(set(s)) < len(s)
    print([k for k in range(243) if ok(k)]) # Michael S. Branicky, Nov 22 2021

A320485 Keep just the digits of n that appear exactly once; write -1 if all digits disappear.

Original entry on oeis.org

0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, -1, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, -1, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, -1, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, -1, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, -1, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, -1, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, -1, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, -1, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, -1, 1, 0, 102, 103, 104, 105, 106, 107, 108, 109, 0, -1, 2, 3, 4, 5, 6, 7, 8, 9, 120, 2, 1, 123, 124, 125, 126, 127, 128, 129, 130, 3
Offset: 0

Views

Author

N. J. A. Sloane, Oct 24 2018

Keywords

Comments

Digits that appear more than once in n are erased. Leading zeros are erased unless the result is 0. If all digits are erased, we write -1 for the result.
The map n -> a(n) was invented by Eric Angelini and described in a posting to the Sequence Fans Mailing List on Oct 24 2018.
More than the usual number of terms are shown in order to reach some interesting examples.
a(n) = -1 mostly. - David A. Corneth, Oct 24 2018

Examples

			1231 becomes 23, 1123 becomes 23, 11231 becomes 23, and 11023 becomes 23 (as we don't accept leading zeros). Note that 112323 disappears immediately and we get -1.
101 and 110 become 0 while 11000 and 10001 become -1.
		

References

  • Eric Angelini, Posting to Sequence Fans Mailing List, Oct 24 2018

Crossrefs

See A320486 for another version.

Programs

  • Maple
    f:= proc(n) local F,S;
      F:= convert(n,base,10);
      S:= select(t -> numboccur(t,F)>1, [$0..9]);
      if S = {} then return n fi;
      F:= subs(seq(s=NULL,s=S),F);
      if F = [] then -1
      else add(F[i]*10^(i-1),i=1..nops(F))
      fi
    end proc:
    map(f, [$0..200]); # Robert Israel, Oct 24 2018
  • Mathematica
    Array[If[# == {}, -1, FromDigits@ #] &@ Map[If[#[[-1]] > 1, -1, #[[1]] ] /. -1 -> Nothing &, Tally@ IntegerDigits[#]] &, 131] (* Michael De Vlieger, Oct 24 2018 *)
  • PARI
    a(n) = {my(d=digits(n), v = vector(10), res = 0, t = 0); for(i=1, #d, v[d[i]+1]++); for(i=1, #d, if(v[d[i]+1]==1, t = 1; res=10 * res + d[i])); res - !t + !n} \\ David A. Corneth, Oct 24 2018
    
  • Python
    def A320485(n):
        return (lambda x: int(x) if x != '' else -1)(''.join(d if str(n).count(d) == 1 else '' for d in str(n))) # Chai Wah Wu, Nov 19 2018

Formula

From Rémy Sigrist, Oct 24 2018: (Start)
a(n) = n iff n belong to A010784.
a(n) <= 9876543210 with equality iff n = 9876543210.
(End)
If n > 9876543210, then a(n) < n. If a(n) < n, then a(n) <= 99n/1000. - Chai Wah Wu, Oct 24 2018

A154566 Smallest 10-digit number whose n-th power contains each digit (0-9) n times, or -1 if no such number exists.

Original entry on oeis.org

1023456789, 3164252736, 4642110594, 5623720662, 6312942339, 6813614229, 7197035958, 7513755246, 7747685775, 7961085846, 8120306331, 8275283289, 8393900487, 8626922994, 8594070624, 8691229761, 8800389678, 8807854905, 9873773268, 8951993472, 9473643936, 9585032094
Offset: 1

Views

Author

Zhining Yang, Jan 12 2009, Jan 13 2009

Keywords

Comments

A number with 10*n digits could contain all ten digits(0-9) n times. The probability of this is (10n)!/((n!)^10 * 10^((10*n)-10^(10*n-1)). There are 10^10-10^(10-1/n)) numbers which are n-th powers of some 10-digit numbers. So there are about (10n)!*(10^10-10^(10-1/n)))/((n!)^10 * 10^((10*n)-10^(10*n-1)) numbers which satisfy the requirements.
Fortunately, I found a larger number than those shown here, for n=26, a(n)=9160395852. Since (10n)!*(10^10-10^(10-1/n))/((n!)^10 * 10^((10*n)-10^(10*n-1)) = 0.31691419..., this is a lucky event!
The sequence is -1 beyond a certain point because when n > 23025850928 we have 9999999999^n < 10^(10*n-1), i.e., it is impossible to obtain a power with 10*n digits. From a(23) to a(600) the only terms which are not -1 are a(24)=9793730157, a(26)=9160395852, a(35)=9959167017, and a(38)=9501874278. - Giovanni Resta, Jan 17 2020

Examples

			For n=18, a(n)=8807854905. That means 8807854905^18 has all digits 0-9 each 18 times and 8807854905 is the smallest 10-digit number which has this property.
		

Crossrefs

Programs

  • Python
    def flag(p, n):
        for i in range(10):
            if not p.count(str(i)) == n:
                return False
        return True
    def a(n):
        for i in range(3*int(10**(10-1/n)/3), 10**10, 3):
            if flag(str(i**n), n):
                return i
    for i in range(1, 41):
        print(a(i), end=", ") # Zhining Yang, Oct 05 2022
  • VBA
    Function Flag(ByVal s As String, ByVal num As Long) As Long
    Dim b&(9), t&, i&
    Flag = 1
    If Len(s) <> 10 * num Then
        Flag = 0
        Exit Function
    End If
    For i = 1 To Len(s)
        t = Val(Mid(s, i, 1))
        b(t) = b(t) + 1
        If b(t) > num Then
            Flag = 0
            Exit Function
        End If
    Next
    End Function
    '
    Function Mypower(ByVal num As Currency, ByVal power As Long) As String
    Dim b(), temp, i&, j&
    ReDim b(1 To 2 * power)
    ReDim s(1 To 2 * power)
    b(2 * power - 1) = Val(Left(num, 5))
    b(2 * power) = Val(Right(num, 5))
    For i = 2 To power
        temp = 0
        For j = 2 * power To 1 Step -1
            temp = b(j) * num + temp
            b(j) = Format(Val(Right(temp, 5)), "00000")
            temp = Int(temp / 10 ^ 5)
        Next
    Next
    Mypower = Join(b, "")
    End Function
    '
    Function a(ByVal n As Long)
    Dim j As Currency, s As String, num&
    For j = 3 * Int(1 + 10 ^ (10 - 1 / n) / 3) To 9999999999# Step 3
        DoEvents
        If Flag(Mypower(j, n), n) = 1 Then
            a = j
            Exit Function
        End If
    Next
    End Function ' Zhining Yang, Oct 11 2022
    

Extensions

Edited by N. J. A. Sloane, Jan 13 2009
Edited by Charles R Greathouse IV, Nov 01 2009
Further edits by M. F. Hasler, Oct 05 2012
a(19)-a(22) from Giovanni Resta, Jan 17 2020
Added escape clause to definition. - N. J. A. Sloane, Nov 22 2022

A115569 Lynch-Bell numbers: numbers n such that the digits are all different (and do not include 0) and n is divisible by each of its individual digits.

Original entry on oeis.org

1, 2, 3, 4, 5, 6, 7, 8, 9, 12, 15, 24, 36, 48, 124, 126, 128, 132, 135, 162, 168, 175, 184, 216, 248, 264, 312, 315, 324, 384, 396, 412, 432, 612, 624, 648, 672, 728, 735, 784, 816, 824, 864, 936, 1236, 1248, 1296, 1326, 1362, 1368, 1395, 1632, 1692, 1764, 1824
Offset: 1

Views

Author

Mike Smith (mtm_king(AT)yahoo.com), Mar 10 2006; also submitted by Andy Edwards (AndynGen(AT)aol.com), Mar 20 2006

Keywords

Comments

This is a subset of some of the related sequences listed below. Stephen Lynch and Andrew Bell are Brisbane surgeons who contributed to the identification of this sequence.
There are 548 Lynch-Bell numbers. A117911 gives the number of n-digit ones. The digit 5 cannot appear in Lynch-Bell numbers containing an even digit; 5 must be the units digit when it appears. The 7-digit Lynch-Bell numbers are 105 permutations of 1289736 (the smallest such). - Rick L. Shepherd, Apr 01 2006
Can be seen/read as a table with row lengths A117911 (rows r > 7 have zero length). - M. F. Hasler, Jan 31 2016

Examples

			384/3 = 128, 384/8 = 48, 384/4 = 96. Thus 384 is Lynch-Bell as it is a multiple of each of its three distinct digits.
		

Crossrefs

Cf. A117911, A117912 (have even digits only), A117913 (have odd digits only), A010784.

Programs

  • Maple
    with(combinat):
    f:= l-> parse(cat(l[])):
    T:= n-> sort(map(f, select(l-> andmap(x-> irem(f(l), x)=0, l),
             map(p-> permute(p)[], choose([$1..9], n)))))[]:
    seq(T(n), n=1..7);  # Alois P. Heinz, Jul 31 2022
  • Mathematica
    Reap[For[n = 1, n < 10^7, n++, id = IntegerDigits[n]; If[FreeQ[id, 0] && Length[id] == Length[Union[id]] && And @@ (Divisible[n, #]& /@ id), Print[n]; Sow[n]]]][[2, 1]] (* Jean-François Alcover, Nov 26 2013 *)
    bnQ[n_]:=Max[DigitCount[n]]==1&&FreeQ[IntegerDigits[n],0]&&Union[Divisible[n,IntegerDigits[ n]]]=={True}; Select[Range[2000],lbnQ] (* Harvey P. Dale, Jun 02 2023 *)
    Cases[Union @@ ((FromDigits@#&/@Flatten[Permutations@# & /@ Subsets[Range@9, {#}], 1])&/@ Range@9), ?(DeleteDuplicates[Divisible[#, IntegerDigits@#]] == {True} &)] (* _Hans Rudolf Widmer, Aug 27 2024 *)
  • PARI
    A115569_row(n)={if(n,my(u=vectorv(n,i,10^i)\10,S=List(),M);forvec(v=vector(n,i,[1,9]),(M=lcm(v))%10==0||normlp(v,1)%3^valuation(M,3)||for(k=1,n!,vecextract(v,numtoperm(n,k))*u%M ||listput(S,vecextract(v,numtoperm(n,k))*u)),2);Set(S),concat(apply(A115569_row,[1..7])))} \\ Return terms of length n if given, else the vector of all terms. The checks M%10 and |v| % 3^v(...) are not needed but reduce CPU time by 97%. - M. F. Hasler, Jan 31 2016
    
  • PARI
    A115569(n)=n>9&&for(r=2,7,(n-=#t=A115569_row(r))>9||return(t[n-9+#t]));n \\ M. F. Hasler, Jan 31 2016
    
  • Python
    def ok(n):
        s = str(n)
        if "0" in s or len(set(s)) < len(s): return False
        return all(n%int(d) == 0 for d in s)
    afull = [k for k in range(9867313) if ok(k)]
    print(afull[:55]) # Michael S. Branicky, Jul 31 2022

Extensions

The full list of terms was sent in by Rick L. Shepherd (see link) and also by Sébastien Dumortier, Apr 04 2006

A154532 a(n) is the largest 10-digit number whose n-th power contains each digit (0-9) n times, or -1 no such number exists.

Original entry on oeis.org

9876543210, 9994363488, 9999257781, 9999112926, 9995722269, 9999409158, 9998033316, 9993870774, 9986053188, 9964052493, 9975246786, 9966918135, 9938689137, 9998781633, 9813743148, 9970902252, 9740383767, 9829440591, 9873773268, 9985819785, 9766102146, 9863817738
Offset: 1

Views

Author

Zhining Yang, Jan 11 2009

Keywords

Comments

A number with 10*n digits may have all ten digits (0-9) repeated n times. The probability of this is (10n)!/((n!)^10 * 10^((10*n)-10^(10*n-1)). There are 10^10-10^(10-1/n)) numbers which are n-th powers of 10-digit numbers. So there may exist Count=(10n)!*(10^10-10^(10-1/n)))/((n!)^10 * 10^((10*n)-10^(2*n-1)) numbers with the desired property.
From a(23) to a(110) the only terms which exist are a(24)=9793730157, a(26)=9347769564, a(35)=9959167017, and a(38)=9501874278. (The other values of a(n) are -1.) - Zhining Yang, Oct 05 2022

Examples

			a(18) = 9829440591, so each digit (0-9) appears 18 times in the decimal expansion of 9829440591^18.
		

Crossrefs

Programs

  • Python
    def flag(p, n):
        b = True
        for i in range(10):
            if not p.count(str(i)) == n:
                b = False
                break
        return b
    def a(n):
        for i in range(10 ** 10 - 1, 3 * int(10 ** (10 - 1 / n) / 3), -3):
            p = str(i ** n)
            if flag(p, n) == True:
                return i
                break
    for i in range(1, 23):
        print(i, a(i))  # Zhining Yang, Oct 10 2022
    
  • Python
    def flag(p, n):
        return all(p.count(d) == n for d in "0123456789")
    def a(n):
        return next(i for i in range(10**10-1,3*int(10**(10-1/n)/3), -3) if flag(str(i**n), n))
    for i in range(2, 23):
        print(i,a(i))  # Michael_S._Branicky, Oct 10 2022

Extensions

Edited by N. J. A. Sloane, Jan 12 2009
a(19)-a(22) from Zhining Yang, Oct 05 2022
Definition revised by N. J. A. Sloane, Nov 22 2022

A073531 Number of n-digit positive integers with all digits distinct.

Original entry on oeis.org

9, 81, 648, 4536, 27216, 136080, 544320, 1632960, 3265920, 3265920
Offset: 1

Views

Author

Zak Seidov, Aug 29 2002

Keywords

Comments

For any base b the number of distinct-digit numbers is finite. For base 10, the maximal distinct-digit number is 9876543210; for any larger number at least two digits coincide. The number of distinct-digit primes is also finite, see A073532.
If "positive" is replaced by "nonnegative" we get the sequence 10, 81, 648, 4536, 27216, 136080, 544320, 1632960, 3265920, 3265920.
Alternatively, if 0 is considered to have 0 digits, one could prefix a(0) = 1. This would be compatible with the given formula and 9/10 rounded to the nearest integer. - M. F. Hasler, Dec 10 2018
a(10) is the final term because no number having more than 10 digits can have all digits distinct. - Jon E. Schoenfield, May 17 2021

Examples

			a(3) = 648 because there are 648 three-digit integers with distinct digits.
		

Crossrefs

Cf. A073532.
Cf. A010784 for the list of these integers.

Programs

  • GAP
    List([1..10],n->9*Factorial(9)/(Factorial(10-n))); # Muniru A Asiru, Dec 11 2018
    
  • Magma
    [9*Factorial(9)/Factorial(10-n): n in [1..10]]; // Vincenzo Librandi, Dec 13 2018
  • Maple
    seq(9*factorial(9)/(factorial(10-n)),n=1..10); # Muniru A Asiru, Dec 11 2018
  • Mathematica
    Table[9*9!/(10-n)!, {n, 10}]
  • PARI
    apply( A073531(n)=if(n<11,9*9!\/(10-n)!), [1..13]) \\ or: 9*binomial(9,10-n)*(n-1)! without need for if(). - M. F. Hasler, Dec 10 2018
    

Formula

a(n) = 9*9!/(10-n)!.

Extensions

Keywords fini, full added by Jon E. Schoenfield, May 17 2021

A137564 a(n) is the number formed by removing from n all duplicate digits except the leftmost copy of each.

Original entry on oeis.org

0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 1, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 2, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 3, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 4, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 5, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 6, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 7, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 8, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 9, 10, 10, 102, 103, 104, 105, 106, 107, 108, 109, 10, 1, 12
Offset: 0

Views

Author

Rick L. Shepherd, Jan 25 2008

Keywords

Comments

Differs from A106612: a(100) = 10, A106612(100) = 100.
Differs from A337864: a(101) = 10, A337864(101) = 101.
a(n)=n iff n is a term of A010784. a(n)A109303.
A010784 is the sequence of distinct terms in this sequence, thus 9876543210 is the largest term here also, as no digit occurs more than once in any given term. Each term except 0 appears infinitely often in this sequence. - Rick L. Shepherd, Oct 03 2020

Examples

			a(100)=10 as a (second) 0 digit is dropped. a(1211323171)=1237.
a(10...1) = 10 for any number of 0's and/or 1's in any order replacing the "..." in the term's index. - _Rick L. Shepherd_, Oct 03 2020
		

Crossrefs

Cf. A106612, A010784 (fixed points), A109303 (non-fixed).
Cf. A043529 (equivalent in binary, except at n=0), A337864.

Programs

A280593 Natural numbers whose digits can be formed by typing adjacent keys on a 123-456-789 keypad without repeating a digit.

Original entry on oeis.org

1, 2, 3, 4, 5, 6, 7, 8, 9, 12, 14, 21, 23, 25, 32, 36, 41, 45, 47, 52, 54, 56, 58, 63, 65, 69, 74, 78, 85, 87, 89, 96, 98, 123, 125, 145, 147, 214, 236, 254, 256, 258, 321, 325, 365, 369, 412, 452, 456, 458, 478, 521, 523, 541, 547, 563, 569, 587, 589, 632, 652, 654, 658, 698, 741, 745, 785, 789
Offset: 1

Views

Author

FUNG Cheok Yin, Jan 06 2017

Keywords

Comments

A subsequence of A010784. - FUNG Cheok Yin, Jul 05 2018

Examples

			The keypad is:
+---+---+---+
| 1 | 2 | 3 |
+---+---+---+
| 4 | 5 | 6 |
+---+---+---+
| 7 | 8 | 9 |
+---+---+---+
It is visibly obvious that 2589 can be formed on the keypad.
		

Crossrefs

Programs

  • Mathematica
    g = Graph[{1 <-> 2, 1 <-> 4,
        2 <-> 1, 2 <-> 3, 2 <-> 5,
        3 <-> 2, 3 <-> 6,
        4 <-> 1, 4 <-> 5, 4 <-> 7,
        5 <-> 2, 5 <-> 4, 5 <-> 6, 5 <-> 8,
        6 <-> 3, 6 <-> 5, 6 <-> 9,
        7 <-> 4, 7 <-> 8,
        8 <-> 5, 8 <-> 7, 8 <-> 9,
        9 <-> 6, 9 <-> 8}];
    f[{a_, b_}] := FindPath[g, a, b, Infinity, All]
    ff = f /@ Flatten[Outer[List, r = Range[9], r], 1];
    A280593 = Sort[Join[r, FromDigits /@ Flatten[ff, 1]]] (* Jean-François Alcover, Jan 06 2017 *)
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