Update: A shorter alternative using FromDigits
with base 10^commonIntegerLength
:
f = FromDigits[#, 10^IntegerLength[#[[1]]]] &;
f @ ConstantArray[3097, 5]
30973097309730973097
Timings: Just in case you need to do this for a large number of long integers, timings for the methods posted so far (fdfid
below is the function FromDigits@Flatten@IntegerDigits@#&
from halirutan's answer):
SeedRandom[1]
n = RandomInteger[10^16];
m = 100000;
t1 = First[RepeatedTiming[r1 = f@ConstantArray[n, m];]];
t2 = First[RepeatedTiming[r2 = catenateInteger@ConstantArray[n, m];]];
t3 = First[RepeatedTiming[r3 = fdfid @ ConstantArray[n, m];]];
t4 = TimeConstrained[First[RepeatedTiming[r4 = myfun[n, m];]], 5];
Grid[{{"function:", "f", "catenateInteger", "fdfid", "myfun"},
{"timing:", 0.0950, 0.169, 0.637, $Aborted}}, Dividers -> All]

r1 == r2 == r3
True
Original answer:
catenateInteger = Composition[FromDigits, StringJoin, IntegerString]
catenateInteger @ ConstantArray[3097, 5]
30973097309730973097
{3097, 3097, 3097} // Map[ToString] // Apply[StringJoin] // ToExpression
, for one. $\endgroup$