Code Sample, a copy-pastable example if possible

import pandas as pd

series = pd.Series(1., pd.period_range('2015', '2016', freq='B'))
resampled = series.resample('3B').sum()

print type(series.index)
print type(resampled.index)

Problem description

A PeriodIndex when resampled by a multiple of BusinessDays becomes a DatetimeIndex.

Expected Output

I would expect it to be a 3B PeriodIndex

Output of pd.show_versions()

INSTALLED VERSIONS ------------------ commit: None python: 2.7.12.final.0 python-bits: 64 OS: Linux OS-release: 4.4.0-36-generic machine: x86_64 processor: x86_64 byteorder: little LC_ALL: None LANG: en_US.UTF-8 LOCALE: None.None pandas: 0.19.2 nose: 1.3.7 pip: 9.0.1 setuptools: 27.2.0 Cython: 0.24 numpy: 1.12.0 scipy: 0.18.1 statsmodels: 0.6.1 xarray: 0.8.2 IPython: 4.1.1 sphinx: None patsy: 0.4.1 dateutil: 2.6.0 pytz: 2016.10 blosc: None bottleneck: 1.2.0 tables: None numexpr: 2.5 matplotlib: 1.5.1 openpyxl: 2.3.3 xlrd: 0.9.4 xlwt: 1.0.0 xlsxwriter: None lxml: 3.5.0 bs4: 4.5.1 html5lib: None httplib2: 0.9.2 apiclient: None sqlalchemy: 1.0.12 pymysql: None psycopg2: None jinja2: 2.8 boto: 2.45.0 pandas_datareader: None

Comment From: chris-b1

Thanks for the report - this is the same issue as #12884, xref #12871 for the master tracker on Period re-sampling. PRs welcome!