cp's OEIS Frontend

A357915 Concatenation of the decimal digits of {n, 1..n}.

%I A357915 #94 Feb 18 2023 20:49:53
%S A357915 11,212,3123,41234,512345,6123456,71234567,812345678,9123456789,
%T A357915 1012345678910,111234567891011,12123456789101112,1312345678910111213,
%U A357915 141234567891011121314,15123456789101112131415,1612345678910111213141516
%N A357915 Concatenation of the decimal digits of {n, 1..n}.
%C A357915 Concatenation of the consecutive integers 1..n, with n prepended (n > 0).
%C A357915 The terms a(1), a(7), a(31), and a(337) are primes (of the form n1...n).
%C A357915 a(3643) is a 13469-digit probable prime (the number of digits is also a prime).
%C A357915 These indices 7, 31, 337, 3643 are themselves primes of the form 6m+1.
%C A357915 For the known terms a(n) which are primes and for a(3643), a(n) == 2 (mod 3).
%C A357915 There is no other prime term for n < 15000 (and no prime term with prime index n < 25000).
%H A357915 Winston de Greef, <a href="/A357915/b357915.txt">Table of n, a(n) for n = 1..367</a>
%H A357915 G. L. Honaker, Jr. and Chris K. Caldwell, <a href="https://primes.utm.edu/curios/page.php/71234567.html">Prime Curios! 71234567</a>
%F A357915 a(n) = concat(n, A007908(n)).
%e A357915 a(2) = 212 since it is the concatenation of the consecutive positive integers <= 2, with 2 prepended.
%t A357915 aUpTo[n_] := Table[ FromDigits @ Flatten @ IntegerDigits @ {i, Range @ i}, {i,n}]; aUpTo[999]
%o A357915 (PARI) a(n) = my(s=Str(n)); for(k=1, n, s=Str(s, k)); eval(s); \\ _Michel Marcus_, Jan 20 2023
%o A357915 (Python)
%o A357915 def a(n): return int(str(n)+"".join(map(str, range(1, n+1))))
%o A357915 print([a(n) for n in range(1, 17)]) # _Michael S. Branicky_, Jan 20 2023
%Y A357915 Cf. A007908, A078257, A172495.
%K A357915 nonn,base,easy
%O A357915 1,1
%A A357915 _Mikk Heidemaa_, Jan 18 2023