The following is a list of companies that have approved stock-buyback programs in the last couple of months.

In addition, these stocks appear to be undervalued when comparing current price to the fair value price, calculated by the Graham Number equation.

The Graham Number is named after a formula developed by legendary value investor Benjamin Graham. It is the maximum price an investor should pay for a stock, and it is derived using only two data points: current earnings per share and current book value per share.

Here's the formula:

The Graham Number = Fair Value of a Stock = Square Root of (22.5) x (Earnings per Share) x (Book Value per Share).

The math of the Graham number is relatively straightforward. It is predicated on the belief that the price-to-earnings (P/EPS) ratio should be no more than 15, and the price-to-book value (P/BVPS) ratio should be no more than 1.5.

From that, the product of the two should not be more than 22.5. In other words, (P/EPS of 15) x (P/BVPS of 1.5) = 22.5, from which the equation was created.

In any case, do you think these companies are undervalued? Management seems to be signaling undervalued conditions, and the Graham equation tends to confirm it. Full details below.

*Interactive Chart: Press Play to compare changes in analyst ratings over the last two years for the top six stocks mentioned below. Analyst ratings sourced from Zacks Investment Research. Note: The numbers on top of items represent the forward P/E ratio, if available.*

The list has been sorted by discount to the Graham number.

**1. Unitrin Inc. (UTR):** Property and Casualty Insurance Industry. Market cap of $1.77B. The company announced the buyback of $300M on Feb-02-11 (represents about 16.96% of current market cap). BVPS at $34.61, diluted EPS at $2.98. Graham number = sqrt(22.5 x $34.61 x $2.98) = $48.17. Current price at $29.17 (implies a potential upside of 39.45%). Short float at 0.63%, which implies a short ratio of 1.75 days. The stock has gained 16.67% over the last year.

** 2. XL Group plc (NYSE:XL):** Property and Casualty Insurance Industry. Market cap of $7.24B. The company announced the buyback of $1.0B on Nov-02-10 (represents about 13.81% of current market cap). BVPS at $32.91, diluted EPS at $1.73. Graham number = sqrt(22.5 x $32.91 x $1.73) = $35.79. Current price at $22.30 (implies a potential upside of 37.69%). Short float at 4.12%, which implies a short ratio of 3.24 days. The stock has gained 21.66% over the last year.

**Accident and Health Insurance Industry. Market cap of $8.29B. The company announced the buyback of $1.0B on Feb-02-11 (represents about 12.06% of current market cap). BVPS at $28.25, diluted EPS at $2.71. Graham number = sqrt(22.5 x $28.25 x $2.71) = $41.50. Current price at $26.01 (implies a potential upside of 37.33%). Short float at 1.4%, which implies a short ratio of 2.15 days. The stock has gained 24.18% over the last year.**

3. Unum Group (NYSE:UNM):

3. Unum Group (NYSE:UNM):

**4. The Travelers Companies, Inc. (NYSE:TRV)**: Property and Casualty Insurance Industry. Market cap of $25.53B. The company announced the buyback of $5.0B on Jan-25-11 (represents about 19.59% of current market cap). BVPS at $58.46, diluted EPS at $6.62. Graham number = sqrt(22.5 x $58.46 x $6.62) = $93.31. Current price at $59.11 (implies a potential upside of 36.66%). Short float at 2.97%, which implies a short ratio of 3.27 days. The stock has gained 14.21% over the last year.

**Information Technology Services Industry. Market cap of $7.34B. The company announced the buyback of $1.0B on Dec-13-10 (represents about 13.63% of current market cap). BVPS at $46.34, diluted EPS at $5.30. Graham number = sqrt(22.5 x $46.34 x $5.30) = $74.34. Current price at $47.84 (implies a potential upside of 35.64%). Short float at 2.16%, which implies a short ratio of 2.17 days. The stock has lost -9.02% over the last year.**

5. Computer Sciences Corporation (NYSE:CSC):

5. Computer Sciences Corporation (NYSE:CSC):

**6. ProAssurance Corporation (NYSE:PRA):**Property and Casualty Insurance Industry. Market cap of $1.93B. The company announced the buyback of $200M on Nov-11-10 (represents about 10.37% of current market cap). BVPS at $58.90, diluted EPS at $6.58. Graham number = sqrt(22.5 x $58.90 x $6.58) = $93.38. Current price at $63.50 (implies a potential upside of 32.00%). Short float at 2.84%, which implies a short ratio of 6.15 days. The stock has gained 16.11% over the last year.

**7. Seagate Technology PLC (NASDAQ:STX)**: Data Storage Devices Industry. Market cap of $5.57B. The company announced the buyback of $2.0B on Nov-29-10 (represents about 35.93% of current market cap). BVPS at $6.13, diluted EPS at $2.40. Graham number = sqrt(22.5 x $6.13 x $2.40) = $18.19. Current price at $12.58 (implies a potential upside of 30.86%). Short float at 2.52%, which implies a short ratio of 1.18 days. The stock has lost -41.50% over the last year.

**8. Healthways Inc. (NASDAQ:HWAY):**Specialized Health Services Industry. Market cap of $480.34M. The company announced the buyback of $60M on Oct-21-10 (represents about 12.49% of current market cap). BVPS at $12.67, diluted EPS at $1.36. Graham number = sqrt(22.5 x $12.67 x $1.36) = $19.69. Current price at $14.32 (implies a potential upside of 27.27%). Short float at 5.5%, which implies a short ratio of 7.47 days. The stock has lost -8.79% over the last year.

**: Education and Training Services Industry. Market cap of $1.44B. The company announced the buyback of $100M on Dec-08-10 (represents about 6.95% of current market cap). BVPS at $15.60, diluted EPS at $1.78. Graham number = sqrt(22.5 x $15.60 x $1.78) = $25.00. Current price at $19.07 (implies a potential upside of 23.71%). Short float at 6.94%, which implies a short ratio of 16.93 days. The stock has lost -5.05% over the last year.**

9. Education Management Corporation (NASDAQ:EDMC)

9. Education Management Corporation (NASDAQ:EDMC)

**10. Aetna Inc. (NYSE:AET):**Health Care Plans Industry. Market cap of $14.94B. The company announced the buyback of $750M on Dec-03-10 (represents about 5.02% of current market cap). BVPS at $25.73, diluted EPS at $4.18. Graham number = sqrt(22.5 x $25.73 x $4.18) = $49.19. Current price at $37.90 (implies a potential upside of 22.96%). Short float at 1.2%, which implies a short ratio of 1.25 days. The stock has gained 20.49% over the last year.

**11. Entergy Corporation (NYSE:ETR):**Electric Utilities Industry. Market cap of $12.82B. The company announced the buyback of $500M on Oct-29-10 (represents about 3.9% of current market cap). BVPS at $47.55, diluted EPS at $6.66. Graham number = sqrt(22.5 x $47.55 x $6.66) = $84.41. Current price at $71.14 (implies a potential upside of 15.72%). Short float at 1.39%, which implies a short ratio of 1.93 days. The stock has lost -5.59% over the last year.

**12. Nanometrics Incorporated (NASDAQ:NANO)**: Scientific and Technical Instruments Industry. Market cap of $378.63M. The company announced the buyback of $10M on Nov-29-10 (represents about 2.64% of current market cap). BVPS at $7.65, diluted EPS at $2.43. Graham number = sqrt(22.5 x $7.65 x $2.43) = $20.45. Current price at $17.54 (implies a potential upside of 14.24%). Short float at 9.56%, which implies a short ratio of 2.93 days. The stock has gained 73.52% over the last year.

**13. Ameron International Corporation (NYSE:AMN):**General Building Materials Industry. Market cap of $625.30M. The company announced the buyback of $50M on Oct-22-10 (represents about 8.0% of current market cap). BVPS at $55.18, diluted EPS at $5.00. Graham number = sqrt(22.5 x $55.18 x $5.00) = $78.79. Current price at $68.16 (implies a potential upside of 13.49%). Short float at 3.32%, which implies a short ratio of 2.81 days. The stock has lost -1.76% over the last year.

**Disclosure:**I have no positions in any stocks mentioned, and no plans to initiate any positions within the next 72 hours.